Fix exception message

buffer->flexFormat is only used if usingRGBAToJpegOverride is true,
which appears to be the uncommon case. As such the unsupported
format exception should be using the writerFormat instead, which is
what's actually being looked up typically.

Bug: 137323248
Test: snippet in bug & compare what format is actually switched on
with what the exception message claims is unsupported.
Change-Id: I54dda0088fd722126e80e38eebe5da2f26378e1d