Better developer experience when using the API

[marcenacp]

List by @mkuchnik:

• Developers shouldn't have to use strings (e.g., "text/csv", "sc:ImageObject").

[mkuchnik]

• Developers should get a meaningful traceback if there is an error (e.g., "failed to join X and Y because Z"). For example, a common error when adding to a Croissant definition iteratively is AssertionError: "GroupRecordSetStart(<record_set>)" should have one and only one predecessor. Got: 0., which can be improved to give a diagnostic of what went wrong.
• Regex semantics should be described (e.g., usage of fnmatch on some operations). Diagnostics on failed matches can also help.