Create a custom has_errors_on RSpec matcher

spec/support/matchers/has_errors_on.rb

@@ -0,0 +1,6 @@
+RSpec::Matchers.define :have_error_on do |expected|
+  match do |actual|
+    expect(actual).to_not be_valid
+    expect(actual.errors.keys).to include expected
+  end
+end