Make a nicer error on regexp failure

Yes, I had a brain fart where I forgot how to write a regexp... but maybe catch RegexpError and just print "Invalid regex" instead of some weird stacktrack like this:

```ruby
borkbork ~/devel/gh/juliandunn/chef-client (master *)$ kitchen test *centos*
-----> Starting Kitchen (v1.0.0.rc.1)
/Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/test-kitchen-1.0.0.rc.1/lib/kitchen/cli.rb:311:in `get_filtered_instances': target of repeat operator is not specified: /*centos*/ (RegexpError)
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/test-kitchen-1.0.0.rc.1/lib/kitchen/cli.rb:297:in `parse_subcommand'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/test-kitchen-1.0.0.rc.1/lib/kitchen/cli.rb:111:in `block in test'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/1.9.1/benchmark.rb:280:in `measure'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/test-kitchen-1.0.0.rc.1/lib/kitchen/cli.rb:107:in `test'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/thor-0.18.1/lib/thor/command.rb:27:in `run'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/test-kitchen-1.0.0.rc.1/lib/kitchen/cli.rb:258:in `invoke_task'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/test-kitchen-1.0.0.rc.1/bin/kitchen:13:in `block in <top (required)>'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/test-kitchen-1.0.0.rc.1/lib/kitchen/errors.rb:81:in `with_friendly_errors'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/test-kitchen-1.0.0.rc.1/bin/kitchen:13:in `<top (required)>'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/bin/kitchen:19:in `load'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/bin/kitchen:19:in `<main>'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/bin/ruby_noexec_wrapper:14:in `eval'
	from /Users/juliandunn/.rbenv/versions/1.9.3-p448/bin/ruby_noexec_wrapper:14:in `<main>'
```