fix rspec-puppet raise_error warning

[jhoblitt]

Resolves this warning:

WARNING: Using the `raise_error` matcher without providing a specific
error or message risks false positives, since `raise_error` will match
when Ruby raises a `NoMethodError`, `NameError` or `ArgumentError`,
potentially allowing the expectation to pass without even executing the
method you are intending to call. Instead consider providing a specific
error class or message. This message can be supressed by setting:
`RSpec::Expectations.configuration.warn_about_potential_false_positives
= false`.

[jenkinsadmin]

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