tools/makemanifest.py: Print nicely formatted errors from mpy-cross.
tools/makemanifest.py: Print nicely formatted errors from mpy-cross.

If mpy-cross exits with an error be sure to print that error in a way that
is readable, instead of a long bytes object.

Signed-off-by: Damien George <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@micropython.org>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython