Bugfix: IDA mnemonics fallback for architectures not supported by capstone

[dominikbayerl]

quokka.CapstoneErrors are currently not handled appropriately, i.e. if you try to get instruction listings for an architecture not supported by capstone, you receive the error quokka.exc.CapstoneError: Unable to find the Architecture:

quokka/bindings/python/quokka/backends/capstone.py

Lines 68 to 71 in 2f1d4cc

	try:
	capstone_arch, capstone_mode = mapping.get(arch)
	except TypeError as exc:
	raise quokka.CapstoneError("Unable to find the Architecture") from exc

This patch addresses this, by handling quokka.CapstoneError with a proper fallback to the IDA disassembly, at least for instruction mnemonics.

[dominikbayerl]

N.B.: I am not entirely sure, if this should better be handled here:
https://github.com/dominikbayerl/quokka/blob/2f1d4ccd708146d02d700e4f1e49f1e57df6dda7/bindings/python/quokka/instruction.py#L171-L182