Tests for wrong parameter data types aren't really needed since Puppet
will do the type checking itself. These tests also caused warnings
because they didn't provide specific error messages to the
`raise_error` matcher.

Also, provide error messages to the remaining `raise_error` matches
that lacked error messages, to avoid warnings like this:

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. Actual error raised was
a Resource Statement, Postfix::... expects a match for Enum['absent',
'present'], got Tuple (line: 5) on node lithium.kenyonralph.com>.
Instead consider provi ding a specific error class or message. This
message can be suppressed by setting:
`RSpec::Expectations.configuration.on_potential_false_positives =
:nothing`. Called from /home/kenyon/git/
puppet-postfix/spec/defines/postfix_virtual_spec.rb:88:in `block (5
levels) in <top (required)>'.