Bug 978837 - Don't show stack trace if --ssh executable not found

rhc app ssh --ssh /path/to/nowhere should return a short error
(not found or not executable) to the user instead of a stack trace.
Merge pull request #418 from ncdc/bz978837-fix-ssh-no-such-file-or-directory-exception

Merged by openshift-bot
If the number of columns is zero, do not wrap.

Addressing
/usr/lib/ruby/gems/1.8/gems/rhc-1.10.1/lib/rhc/core_ext.rb:100:in `textwrap_ansi': can't do {n,m} with n > m: / (RegexpError)
      (
        # Match substrings that end in whitespace shorter than limit
        (?:(?:\e\[\d+(?:;\d+)*[@-~])+.?|.(?:\e\[\d+(?:;\d+)*[@-~])*){1,0}
        (?:\s|$)                     # require the limit to end on whitespace
        |
        # Match all continguous whitespace strings
        (?:(?:\e\[\d+(?:;\d+)*[@-~])+.?|.(?:\e\[\d+(?:;\d+)*[@-~])*)+?
        (?:\s|$)
        (?:\s+|$)?
      )
      /
	from /usr/lib/ruby/gems/1.8/gems/rhc-1.10.1/lib/rhc/highline_extensions.rb:40:in `say'
	from /usr/lib/ruby/gems/1.8/gems/rhc-1.10.1/lib/rhc/commands/cartridge.rb:63:in `list'
	from /usr/lib/ruby/gems/1.8/gems/rhc-1.10.1/lib/rhc/highline_extensions.rb:177:in `call'
	from /usr/lib/ruby/gems/1.8/gems/rhc-1.10.1/lib/rhc/highline_extensions.rb:177:in `section'
	from /usr/lib/ruby/gems/1.8/gems/rhc-1.10.1/lib/rhc/highline_extensions.rb:191:in `paragraph'