Make gocloc continue when encountering directory read errors

[k00l-beanz]

As of right now, gocloc does not return any results if it encounters a directory it cannot read.

Here's my small PoC.

1. Gocloc scanning a clean clone of 'thefuck' repository: https://github.com/nvbn/thefuck/

   $ ./gocloc ~/Documents/Repos/test_cases/gocloc/tests/thefuck/
   Results:
   -------------------------------------------------------------------------------
   Language                     files          blank        comment           code
   -------------------------------------------------------------------------------
   Python                         404           3591            733          11981
   Markdown                         3            138              0            541
   JSON                             1              3              0             36
   YAML                             1              3              0             22
   Plain Text                       1              0              0             11
   Bourne Shell                     1              1              0              3
   -------------------------------------------------------------------------------
   TOTAL                          411           3736            733          12594
   -------------------------------------------------------------------------------

2. Change permissions of 'tests' directory

   # ls -la 
   drwxrwxr-x 8 test test   4096 Sep 13 11:46 tests
   # chown root:root tests
   # chmod 600 tests
   $ cd tests
   -bash: cd: tests/: Permission denied
   $ ./gocloc ~/Documents/Repos/test_cases/gocloc/tests/thefuck/
   fail gocloc analyze. error: open /home/maximillian/Documents/Repos/test_cases/gocloc/tests/thefuck/tests: permission denied

   It would be nice to have gocloc continue upon read errors. Something like:

   $ ./gocloc ~/Documents/Repos/test_cases/gocloc/tests/thefuck/
   open /home/test/Documents/Repos/test_cases/gocloc/tests/thefuck/tests: permission denied
   -------------------------------------------------------------------------------
   Language                     files          blank        comment           code
   -------------------------------------------------------------------------------
   Python                         209           1630            470           4658
   Markdown                         3            138              0            541
   JSON                             1              3              0             36
   YAML                             1              3              0             22
   Plain Text                       1              0              0             11
   Bourne Shell                     1              1              0              3
   -------------------------------------------------------------------------------
   TOTAL                          216           1775            470           5271
   -------------------------------------------------------------------------------

   Which now shows which directories are denied.

Not a problem with small projects. When using gocloc on larger projects this can cause gocloc to hang for a while before erroring out.

Sorry for hacky solution. I am not proficient in Go.

[hhatto]

Thanks for your contribution 👍