Unfriendly error when template file is missing

[simpsora]

I had a typo in the filename of my template file in stack_master.yml, which meant that it referred to a nonexistent file (left off the .rb extension):

    elb-test:
      template: elb_test

The error message was quite confusing, however:

$ stack_master apply us-east-1 elb-test
Executing apply on elb-test in us-east-1
StackMaster::TemplateCompiler::TemplateCompilationFailed Failed to compile /Users/.../templates/elb_test.
 Caused by: KeyError key not found: :""

Note the trailing . on the template filename (missing from my definition), and the resulting KeyError with a weird value (:"").

Ideally it would raise a more descriptive error here.

$ stack_master --version
StackMaster 0.7.1