rabbit_reader: Format `Explanation` before passing it

... to `rabbit_misc:amqp_error()`.

`Explanation` can be a user input. Therefore, we don't want to pass it
as a format string to `rabbit_misc:amqp_error()`, as we can't trust its
content.

Now, we pass our own format string ("~s") and `Explanation` becomes
the argument to that format string. This ensures we don't interpret
untrusted user input.

Discussed with: @dcorbacho, @essen and @michaelklishin.

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