Update conversion error in convert function

[gadhagod]

Changed the error from AbstractType.convert() a little to be more descriptive. When you have hundreds of objects to convert, it makes life easier for me by raising the error with the value of the object that it failed to convert. For example,
Instead of:

ValueError: failed to convert from str to DATETIME

It would raise:

ValueError: failed to convert 2021-02-10T99:59:00+0800 from str to DATETIME

Now I know exactly what needs to be fixed.

[codecov]

Codecov Report

Merging #2 (8ca54fb) into master (f7a059c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #2   +/-   ##
=======================================
  Coverage   94.82%   94.82%           
=======================================
  Files          50       50           
  Lines         909      909           
  Branches       45       45           
=======================================
  Hits          862      862           
  Misses         41       41           
  Partials        6        6           

Impacted Files	Coverage Δ
typepy/type/_base.py	90.74% <ø> (ø)

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[coveralls]

Coverage remained the same at 95.49% when pulling 8ca54fb on gadhagod:master into f7a059c on thombashi:master.

[thombashi]

Thank you for your pull request.