Print process mailbox on failing assert_receive

After almost one year (https://github.com/elixir-lang/elixir/pull/1728), that's my second attempt to make assert_receive(d) error messages better.

When assert_receive fails we don't know if we have any message received or we just matched the wrong pattern. My proposal is: print the mailbox of the process.

Is it something desirable?
Indentation inside the function is off (should be two chars).