[JITTER] Error codes from *append instructions

[Jose E. Marchesi]

Hi people!

There is a problem with the *append routines that Jitter generates: they
will abort on certain circumstances, like when an instruction is
appended and some of its parameters are missing, or when an invalid
instruction name is provided.

This makes it impossible to use Jitter in a library like libpoke.

So we are requesting for these routines to return error codes instead of
aborting.  We should be able to distinguish each erroneous situation so
we can wrapper the jitter calls properly in our library.

Thanks! :)