👌 Improve warnings for invalid filters (add source location and subtype)

[chrisjsewell]

See tests/test_filter.py, e.g.

srcdir/index.rst:51: WARNING: Filter 'xxx' not valid. Error: name 'xxx' is not defined. [needs.filter]

[codecov]

Codecov Report

Attention: Patch coverage is 82.60870% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 85.90%. Comparing base (eb81f2e) to head (07a12d5).

Files	Patch %	Lines
sphinx_needs/roles/need_count.py	33.33%	2 Missing ⚠️
sphinx_needs/directives/needgantt.py	75.00%	1 Missing ⚠️
sphinx_needs/filter_common.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1128      +/-   ##
==========================================
+ Coverage   85.81%   85.90%   +0.08%     
==========================================
  Files          56       56              
  Lines        6505     6511       +6     
==========================================
+ Hits         5582     5593      +11     
+ Misses        923      918       -5     

Flag	Coverage Δ
pytests	85.90% <82.60%> (+0.08%)	⬆️

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