CA-85776: Improve parsing of bootloader errors.

eliloader should now always throw a RuntimeError in line with pygrub,
so we now look for that in the stacktrace and raise the rest of the
stacktrace as an error message.

Signed-off-by: John Else <john.else@citrix.com>
Signed-off-by: David Scott <dave.scott@eu.citrix.com>