rename error message step definition

the step definition asked for flash error messages
but it actually tested for .errorExplaination messages.

This is why I renamed it to just ask for error messages

Conflicts:
	features/step_definitions/error_steps.rb
we can now ask for _no_ error message in our cukes
combined two flash message step definitions, which can now be negated
add cuke for testing the removal of success messages when a member was added
only include members_form.js once
remove flash messages too, when changing members. execute the remove script on every submit
add entry for #1488 in CHANGELOG

Conflicts:
	doc/CHANGELOG.md
we have to render something even if the member is invalid
fix member role_empty validation

because add_on_empty was used, it failed to invalidate the member object
if there all roles are marked for deletion (but are still present)
add spec for non-empty roles validation in Member
add CHANGELOG entry, because we have fixed #1505
fix typo in step definition
Merge branch 'feature/rails3_do_not_remove_member_when_last_role_was_removed' into feature/rails3
Merge branch 'feature/rails3_remove_old_member_create_messages' into feature/rails3