Improve exception handling for not having a tourbus.yml file

[base10]

Hi,

I very recently wrote my first tour to evaluate the tool. I wasn't using any variable substitution, so I didn't think a tourbus.yml file was necessary.

I ended up with the following stack trace:

I'm trying @bbozo's fork with 1.9.3-p392 and getting failures.

/Users/nathan/.rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in require': iconv will be deprecated in the future, use String#encode instead. /Users/nathan/.rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/tourbus-2.0.2/bin/tourbus:8:in': undefined method symbolize_keys' for false:FalseClass (NoMethodError) from /Users/nathan/.rbenv/versions/1.9.3-p392/bin/tourbus:23:inload' from /Users/nathan/.rbenv/versions/1.9.3-p392/bin/tourbus:23:in `'

It was pointed out to me that not having a tourbus.yml would present that issue.

I propose rescuing that exception and reraising with a more descriptive error message.

Thanks!