Add return codes to pflogger

[tclune]

When a format is bad pflogger crashes with a minimally helpful message. Preferable would be to throw an exception, but since we do not have those in Fortran, we must use the Fortran convention of an optional return code.

The fix is straightforward, but rather tedious to propagate. It may also require analogous changes in gFTL, though RC's there should mostly be in place already.