Currently, if you have an error in a payload module (e.g. you have a syntax
error in a module), the error is not noticed early enough in the generation process by the parameter validator, leading to a mysterious message like so:
```
Error: undefined method `platform' for nil:NilClass
```
This change cleans up some of the error handling, checks to see if the payload
module can be instantiated, and gives a more useful error output for the
different cases. This also tweaks some of the literal string outputs to match
other exception strings from payload_generator.
Brent Cook
d988fe6e5c