Check the return value of all emulops in the emulation code, and abort

tty output as soon as we hit a failure.

Since the `output' of a character may cause several emulops to be called
(e.g. if it causes scrollup or if this is the end of an escape sequence),
all emulation code maintain a so-called `abort state', to be able to properly
recover when the character is tentatively output later, and not reissue
the emulops which did not fail the first time.

With help from mglocker@

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