better error message for TypeError on datetime parsing

[samuelcolvin]

Change Summary

While working on #763 I discovered that we had no tests and ugly errors if anything other than a native datetime/date/time/timedelta, string, int or float was passed to those parsers.

Changes:

• fix the above - more descriptive error message
• add support for bytes by decoding them before regex matching
• change invalid values to be value_errors not type_errors - more correct.

Github doesn't do a very good job of showing it, but this change is mainly just an indentation change plus adding if isinstance(value, bytes): value = value.decode().

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.rst file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #766 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #766   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines        2734   2747   +13     
  Branches      541    546    +5     
=====================================
+ Hits         2734   2747   +13

Impacted Files	Coverage Δ
pydantic/errors.py	100% <100%> (ø)	⬆️
pydantic/datetime_parse.py	100% <100%> (ø)	⬆️

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