Fix ParameterMissingError message

It referred to an instance variable instead of the block variable
causing this error to return a very confusing message.

I have looked through the tests and see there are no specific tests for
the error messages. Should it be added?

Perhaps just something that checks that the key is contained within the
error message.
Add failing test for fix

I figured this might be an okay trade-off to reuse the same method, but
still be able to pass in a string or regular expression to fully or
partially match error message content.
Merge pull request #8 from terlar/fix/parameter-missing-error-message

Fix ParameterMissingError message