Wrap Vim-called methods in a error logging guard
Wrap Vim-called methods in a error logging guard

When an unrescued Ruby exception is raised Vim prints the exception text
but not the backtrace, yielding reports like this one:

> Error detected while processing function commandt#CommandTShowFileFinder:
> line    2:
> TypeError: can't convert nil into String

(Source: https://github.com/wincent/command-t/issues/146)

This commit wraps all Vim-called methods (ie. all messages sent to
`$command_t` in autoload/commandt.vim) in a guard that catches
exceptions, prints the stacktrace and then re-raises them. This should
yield more useful output; for example, the above would become something
like:

> ~/.vim/bundle/command-t/ruby/command-t/controller.rb:59:in `+'
> ~/.vim/bundle/command-t/ruby/command-t/controller.rb:59:in `show_file_finder'
> Error detected while processing function commandt#CommandTShowFileFinder:
> line    2:
> TypeError: can't convert nil into String

We trim off the two bottom frames ("main", and the guard).

vrkansagara/command-t,lencioni/command-t,supriyantomaftuh/command-t,yrro/command-t,wincent/command-t,lencioni/command-t,mtimkovich/command-t,mtimkovich/command-t,besarthoxhaj/command-t,wincent/command-t,vrkansagara/command-t,besarthoxhaj/command-t,supriyantomaftuh/command-t,wincent/command-t,yrro/command-t