Fix negative assertion for custom span error matching

[marcotc]

This PR fixes the negative matching case for our custom error matchers, e.g. have_error_message matcher (is_expected.to_not have_error_message).

There was one test that had an incorrect assertion: http.rb

dd-trace-rb/spec/ddtrace/contrib/httprb/instrumentation_spec.rb

Lines 152 to 154 in 774ee8e

	it 'has no error set' do
	expect(span).to_not have_error_message
	end

This assertion is for the 404 case, which by default is considered an error by our integration. The tracer itself is correctly recording this error, but the test was broke, and was actually asserting that there was no error.