Custom exceptions

[sadielbartholomew]

cfdm does not use any custom exceptions, other than DeprecationError in mixin.netcdf. We could likely make some classes a bit cleaner & improve user feedback on errors etc. by creating & applying some custom exception classes for which to delegate some error handling.

(Copied, minus noise & with a little tidying, from #64 (comment))

Ideas

I can flesh this out further e.g. with some potential inheritance structure as we think about it & work out what might be useful, but firstly to record some potential candidates for useful exceptions:

• cfdm implementation errors (relating to attempts to subclass);
• CF compliance warnings (as specific forms of Python warnings lib warnings that the user can opt to enable or disable by interfacing with the logging framework);
• construct i.e. metadata errors:
  • missing i.e. not defined when expected/required;
  • mismatched dimensions or shape;
  • invalid;
  • etc.
• data errors, e.g perhaps:
  • array-related;
  • compression-related;
• field construct operation errors:
  • on reading fields;
  • on writing fields;
  • on aggregation;
• errors related to a specific encoding, notably netCDF.