Missing entries from `errors.json`

[paddybyers]

errorsHelp.json contains some codes that do not have corresponding entries in errors.json:

400
500
50305
61002
91200 (this redirects to 90007)

[mattheworiordan]

We should have a build step that fails the build if this is the case. Trivial to implement and will remove the need for us to manually avoid this in future. Could incorporate idea in #161 as well. WDYT?