Don't halt upon error on a single gem

[ixti]

Gem backports has directory named tags. Thus gem ctags halts (and produces very cryptic error):

% gem ctags
Generating ctags for  abstract-1.0.0
Generating ctags for  actionmailer-3.0.20
Generating ctags for  actionpack-3.0.20
Generating ctags for  activemodel-3.0.20
Generating ctags for  activerecord-3.0.20
Generating ctags for  activeresource-3.0.20
Generating ctags for  activesupport-3.0.20
Generating ctags for  addressable-2.3.5
Generating ctags for  addressable-2.3.3
Generating ctags for  ansi-1.4.3
Generating ctags for  api_cache-0.2.3
Generating ctags for  arel-2.0.10
Generating ctags for  autoparse-0.3.3
Generating ctags for  awesome_print-1.1.0
ERROR:  While executing gem ... (Errno::EISDIR)
    Is a directory - tags

This patch does not halts, instead it just reports an error:

Failed processing ctags for backports-3.3.3:
  (Errno::EISDIR) Is a directory - tags
���```

[ixti]

Although my initial problem was kindly solved by @marcandre, I still think it's a good thing to not halt if errro happened in the middle. Or at least it should report which gem caused an error.

[bruno-]

Hi, the same error just happened to me and left me in a clueless state..
It would be great if this is fixed

Edit: I just checked and I also have an error with backports gem..

[ixti]

/cc @tpope can you please merge it or, I can update it to fit your requirements if something you don't like...

[tpope]

Squash in a fix for the inadvertent whitespace change and I'll merge and cut a release. Thanks!

[ixti]

@tpope Sorry about that whitespace mess (was using GH interface for editing). Now it's fixed.

[ixti]

Awesome! Thanks!