Parsing exception

[kmvanbrunt]

Fixes #905

Before we merge this, I need to update test_debug_not_settable() since missing a quote no longer calls pexcept().

We also need a unit test to make sure postcmd hooks aren't called if argparse fails.

I have 2 questions:

1. Is CmdLineError the right name for this exception? My docstring says it's an error that occurs during parsing. But that's slightly misleading since StatementParser already parsed the command line. CmdLineErrors are raised by our argparse decorators which actually parse after postparsing hooks.

2. In onecmd_plus_hooks, should I remove "Invalid Syntax: " before the shlex error? I'm assuming that ValueError will always be a syntax error, but that may not be correct.

[codecov]

Codecov Report

Merging #906 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #906   +/-   ##
=======================================
  Coverage   97.44%   97.45%           
=======================================
  Files          15       15           
  Lines        3683     3690    +7     
=======================================
+ Hits         3589     3596    +7     
  Misses         94       94           

Impacted Files	Coverage Δ
cmd2/cmd2.py	96.67% <100.00%> (-0.01%)	⬇️
cmd2/decorators.py	100.00% <100.00%> (ø)
cmd2/exceptions.py	100.00% <100.00%> (ø)
cmd2/parsing.py	100.00% <100.00%> (ø)

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