Make error messages consistent

[jcouball]

Change 1:

Error messages should consistently use the "expected ... but it ..." format.

Reviewed all error messages in the project.

Change 2:

Changed all tests that have error message expectations from:

expect { subject }.to raise_error(ArgumentError, expected_message)

to:

expect { subject }.to raise_error(ArgumentError) do |error|
  expect(error.message).to match(expected_message)
end

so that error messages are diff'd when they don't match.