show original text in editor when reediting

before we only show the error messages returned from server, which is bad for
user to correct.

I also tweaked error messages a bit so it won't affect the upcoming
submission, e.g. error messages from server could be:

    # Could not create ticket.
    # Foo: Unknown field

    Foo: bar

after tweaking, it becomes:

    # Could not create ticket.
    # Foo: Unknown field
    #
    #     Foo: bar

see also #22287
Ensure a newline is between the error comments and the previously submitted form
Merge branch '4.0/rest-reedit-content' into 4.0-trunk