Fallback to text lexer when lexer not found

[darrenburns]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

Catches an exception raised inside pygments by guess_lexer_for_filename in the event that no lexer could be found. When this happens, we now fall back to the TextLexer.

Closes #2018

[codecov-commenter]

Codecov Report

Merging #2133 (b777793) into master (7105781) will decrease coverage by 3.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2133      +/-   ##
==========================================
- Coverage   99.46%   96.46%   -3.01%     
==========================================
  Files          72       72              
  Lines        7339     7415      +76     
==========================================
- Hits         7300     7153     -147     
- Misses         39      262     +223     

Flag	Coverage Δ
unittests	96.46% <100.00%> (-3.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
rich/__main__.py	100.00% <ø> (ø)
rich/console.py	97.52% <100.00%> (-1.63%)	⬇️
rich/terminal_theme.py	100.00% <100.00%> (ø)
rich/traceback.py	99.55% <100.00%> (+<0.01%)	⬆️
rich/_windows_renderer.py	0.00% <0.00%> (-100.00%)	⬇️
rich/_win32_console.py	3.19% <0.00%> (-88.30%)	⬇️

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