Empty statement visibility adjustment

[tleonhardt]

Moved custom cmd2 exceptions to a separate file and removed them from public API.

Closes #889

[codecov]

Codecov Report

Merging #893 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
- Coverage   97.46%   97.43%   -0.04%     
==========================================
  Files          14       15       +1     
  Lines        3674     3669       -5     
==========================================
- Hits         3581     3575       -6     
- Misses         93       94       +1     

Impacted Files	Coverage Δ
cmd2/rl_utils.py	100.00% <0.00%> (ø)	⬆️
cmd2/clipboard.py	100.00% <0.00%> (ø)	⬆️
cmd2/cmd2.py	96.66% <0.00%> (-0.01%)	⬇️
cmd2/utils.py	97.89% <0.00%> (-0.27%)	⬇️
cmd2/plugin.py	100.00% <0.00%> (ø)	⬆️
cmd2/history.py	100.00% <0.00%> (ø)	⬆️
cmd2/parsing.py	100.00% <0.00%> (ø)	⬆️
cmd2/exceptions.py	100.00% <0.00%> (ø)

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