update docs and wrap pebble.ExecError with custom error

[Thanhphan1147]

This PR adds missing commits from #620

• Add missing custom exception wrapping pebble.ExecError for mas-cli commands
• Update docs about mjolnir

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Manging Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex)
• The changelog is updated with changes that affect the users of the charm.

[javierdelapuente]

Will this exception be catched somewhere? Otherwise I think the unit will go into error state (and currently synapse does not go into error state, but into blocked state I think)

[Thanhphan1147]

My reasoning is that if this command fails then there's something wrong with the postgresql database which should put the charm into an error state as there's usually no specific action for the operator to take

[javierdelapuente]

The issue with getting into error state is that it is problematic.

For example, as you are not using the latest version of the redis library, if you get into error state and the redis url changes (because for example the redis pod was recreated), the charm will not get notified, and you may not get out of that issue (as has happened in the discourse charm). Updating the redis library will fix that, but other issues may arise, as not being able to upgrade the charm in that state...

[amandahla]

Should the exception description be the same as the logger.exception?

[amandahla]

Same here regarding expcetion description and logger.exception

[github-actions]

Test coverage for 24e3c7e

Name                                    Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------------------------------
src/auth/__init__.py                        0      0      0      0   100%
src/auth/mas.py                            78      6      2      1    91%   82-84, 101-103, 136->138
src/backup.py                             175      5     20      2    96%   353-354, 423-424, 481->483, 484
src/backup_observer.py                    134     16     12      0    89%   132-135, 140-143, 179-182, 211-214
src/charm.py                              272     18     64     10    92%   141->143, 146, 271, 275-276, 282-283, 304-305, 329, 336, 411-415, 418-419, 447-449, 469
src/charm_types.py                         30      0      0      0   100%
src/database_client.py                     57      1      8      4    92%   35, 47->exit, 69->exit, 88->98
src/database_observer.py                   49      4      2      0    92%   61-64
src/exceptions.py                           3      0      0      0   100%
src/matrix_auth_observer.py                69      7     10      2    89%   66, 146, 160-164
src/media_observer.py                      45      4      2      1    89%   60-62, 81
src/observability.py                       14      0      0      0   100%
src/pebble.py                             194     23     44     11    86%   157->exit, 168-172, 216-217, 237-238, 256-259, 320->325, 330-331, 343-344, 346-347, 378, 380, 382, 384, 386, 417
src/redis_observer.py                      39      3      4      0    93%   63-66
src/s3_parameters.py                       22      0      4      0   100%
src/smtp_observer.py                       61      1     14      2    96%   89, 108->113
src/state/__init__.py                       0      0      0      0   100%
src/state/charm_state.py                  127      9     32      7    90%   164, 168, 189, 214, 220, 226, 230-231, 314
src/state/mas.py                           78      8      6      3    87%   152, 158-159, 183-185, 200, 220
src/state/validation.py                    36      3      2      0    92%   108-110
src/synapse/__init__.py                     3      0      0      0   100%
src/synapse/api.py                        129      6     18      4    93%   111-112, 161, 172, 174, 314
src/synapse/workload.py                   118      4     24      0    94%   365-368
src/synapse/workload_configuration.py     149     26     34     12    79%   90->exit, 94-95, 143-144, 173, 193-194, 226-227, 260, 269-270, 285, 290-291, 312-313, 332->337, 338, 356->358, 368-369, 397, 405->407, 407->409, 414-415, 435->442, 445, 465-466
src/user.py                                23      0      2      0   100%
-----------------------------------------------------------------------------------
TOTAL                                    1905    144    304     59    91%

Static code analysis report

Working... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:00
Run started:2025-01-15 13:11:43.079494

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 9723
  Total lines skipped (#nosec): 3
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):