Bug 1091304 - Improve fmp4 logging. r=kinetik
The %#x format specifier for hex values doesn't work with our
logging macros, and MacOS error codes are generally listed
in decimal anyway.

Also log the reason for a Drain() call to assist debugging.
Bug 1091304 - Fix code style. k=kinetik
We like braces around conditional bodies, and in fact this tripped
me up editing the code.