input: improve setting/property validation errors

[jkloetzke]

The validation of internal settings already produces proper schema exceptions if errors are found. We must not cat these exceptions but propagate them, though.

Likewise, allow external plugins to throw schema.SchemaError exceptions. In case the validate() function returns false, throw the exception ourselves.

Fixes #696.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.25%. Comparing base (a7e2791) to head (b461023).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pym/bob/input.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #697   +/-   ##
=======================================
  Coverage   89.25%   89.25%           
=======================================
  Files          50       50           
  Lines       16443    16446    +3     
=======================================
+ Hits        14676    14679    +3     
  Misses       1767     1767           

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