Improve error reporting

[JJ]

If you run documentable outside the root directory, it returns:

Checking for changes...
Source verification failed with:
doc is not a directory
  in submethod TWEAK at /home/jmerelo/.rakudobrew/versions/moar-2019.07.1/install/share/perl6/site/sources/D8380CC32DB7035B76CD2282A9EF7BCE5986F884 (Pod::To::Cached) line 180
  in sub MAIN at /home/jmerelo/.rakudobrew/versions/moar-2019.07.1/install/share/perl6/site/sources/2D9DBDEC1292D3B5D1A752CD491B5232A0322942 (Perl6::Documentable::CLI) line 207
  in sub RUN-MAIN at /home/jmerelo/.rakudobrew/versions/moar-2019.07.1/install/share/perl6/site/sources/2D9DBDEC1292D3B5D1A752CD491B5232A0322942 (Perl6::Documentable::CLI) line 21
  in block <unit> at /home/jmerelo/.rakudobrew/versions/moar-2019.07.1/install/share/perl6/site/resources/37E70165280CF8465DA5E204F97C6395054B9837 line 3
  in sub MAIN at /home/jmerelo/.rakudobrew/bin/../versions/moar-2019.07.1/install/share/perl6/site/bin/documentable line 3
  in block <unit> at /home/jmerelo/.rakudobrew/bin/../versions/moar-2019.07.1/install/share/perl6/site/bin/documentable line 1

Capturing the error and saying exactly what happens (it does not find the doc directory) might be a better option.

[antoniogamiz]

I will fix this, though until finanalyst/pod-cached#22 is fixed, it could be misleading.