src/bootchooser: adapt custom script subprocess launch error messages
src/bootchooser: adapt custom script subprocess launch error messages

The error messages for failed execution of the custom bootloader backend
handler script should look like the ones for any other issue. The most
important here is to provide a readable message concatenation for what
we need to end with a colon followed by a space.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

rauc/rauc,jluebbe/rauc,rauc/rauc,jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc