Have better internal error types

[iainlane]

What we do at the minute is mainly use fmt.Errof to add a bit of context to our errors. We could be a bit better about that, and add our own error types.

Likely we could use those to print better error messages, but the main benefit would be in the testsuite we could check the error returned to see if the condition we wanted has happened - currently we check it in a pretty naive way.

[willisxd213]

im not sure what im doing im new to all this

[iainlane]

This isn't likely to be an issue which is suitable for someone completely new to programming, rather someone who knows how to do software engineering somewhat already but is trying to get up to speed on the project.