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