Update error message for enum validator

[dmontagu]

Change Summary

Modify the EnumError message to be a little clearer (in line with the WrongConstantError).

Related issue number

#671

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[dmontagu]

putting the type in the context (rather than the permitted values, as in WrongConstantError), means that the type is visible in the traceback for debugging purposes, while only the permitted values are visible in the error message (which may be served externally).

[samuelcolvin]

makes sense, the idea of ctx was something like "things you might need when rendering your own error message".

[codecov]

Codecov Report

Merging #673 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #673   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines        2712   2716    +4     
  Branches      537    538    +1     
=====================================
+ Hits         2712   2716    +4

[samuelcolvin]

LGTM, will merge when I get this far through 😓

[MrMrRobat]

Why don't just add kwargs support to context manager?
and then:

with change_exception(errors.EnumError, ValueError, enum_type=field.type_):
    ...

[dmontagu]

I'm guessing that would be better placed in a separate PR; not sure it would be worth it if it was only used in this instance. But I haven't looked to see if it could simplify the error handling anywhere else.