Move answer result messages to its own place

Problem/Motivation

It's not really optimal to use drupal_set_message() for delivering the feedback from answer evaluations – the message should be presented at the same place as the question and not risk being confused with system messages.

Proposed resolution

Have the result be a part of the actual $form array, and presented above the question.

Remaining tasks

Investigate how $form_state might be used to do this. Maybe this would require that evaluation is done on form validate? If so, this should be solved at the same time as #1827898: On incorrect answer: Keep answer on page reload.

User interface changes

(As described in the problem/motivation.)

API changes

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