Fix #6497: custom lexers fails highlighting when syntax error

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• refs: 2.1.0 problem with extending the lexers dictionary #6497

[codecov]

Codecov Report

Merging #6502 into 2.1.2 will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##            2.1.2    #6502   +/-   ##
=======================================
  Coverage   83.65%   83.65%           
=======================================
  Files         280      280           
  Lines       40478    40478           
  Branches     5968     5968           
=======================================
  Hits        33862    33862           
  Misses       5292     5292           
  Partials     1324     1324

Impacted Files	Coverage Δ
sphinx/highlighting.py	86.23% <100%> (ø)	⬆️

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