Installd: Add error message to dexopt

To give better messages to the binder call, add an out parameter.
This is only the start, threading things further into the calls
dexopt makes, and formatting according to exit codes, will be in
a follow-up CL.

Test: installd_dexopt_test
Change-Id: I0271212c5cddf3a6317d6b597e97537865723fdc