feature(i18n): allows passing multiple message keys to elgg_echo()

When an array of keys are passed, it looks for each translation (including
English versions) and returns false if none are found. No notices are
emitted.

Fixes #6299

/cc @ewinslow @juho-jaakkola @jdalsem 
The body of this if statement always returns without dropping down to the logic below. That makes me think this should actually be a new function. `elgg_echo_with_fallbacks` where `_with_fallbacks` is bikesheddable.