handle non-urlencoded commas with selection "|=" OR filters

[bjamesvERT]

The web API defaults to a secure (and generic/unclear) HTTP 500 error when a Selection "|=" filter is typed with a non-URLEncoded comma.

API should recognize the syntax Exception and display a specific, informative HTTP 400 error - instructing the user that "|=" filter is the problem (and possibly how to correct it)

Example query: http://www.nwfsc.noaa.gov/data/api/v1/source/trawl.catch_fact/selection.json?filters=date_dim$year%7C=[%221999%22,%222000%22]

[bjamesvERT]

The internally-logged (secure) Apache error message:

[Wed Jun 08 08:03:36 2016] [error] [client {IP_REMOVED}]     str_values = json.loads( str_right)
[Wed Jun 08 08:03:36 2016] [error] [client {IP_REMOVED}]   File "/var/www/wsgi-scripts/warehouse/miniconda3/envs/warehouse-env/lib/python3.4/site-packages/simplejson/__init__.py", line 516, in loads
[Wed Jun 08 08:03:36 2016] [error] [client {IP_REMOVED}]     return _default_decoder.decode(s)
[Wed Jun 08 08:03:36 2016] [error] [client {IP_REMOVED}]   File "/var/www/wsgi-scripts/warehouse/miniconda3/envs/warehouse-env/lib/python3.4/site-packages/simplejson/decoder.py", line 370, in decode
[Wed Jun 08 08:03:36 2016] [error] [client {IP_REMOVED}]     obj, end = self.raw_decode(s)
[Wed Jun 08 08:03:36 2016] [error] [client {IP_REMOVED}]   File "/var/www/wsgi-scripts/warehouse/miniconda3/envs/warehouse-env/lib/python3.4/site-packages/simplejson/decoder.py", line 400, in raw_decode
[Wed Jun 08 08:03:36 2016] [error] [client {IP_REMOVED}]     return self.scan_once(s, idx=_w(s, idx).end())
[Wed Jun 08 08:03:36 2016] [error] [client {IP_REMOVED}] simplejson.scanner.JSONDecodeError: Expecting ',' delimiter or ']': line 1 column 8 (char 7)```

[bjamesvERT]

non-URLEncoded comma should either be supported, or give a better error message