Read using a file handle instead of read() method

The read() method silently ignores errors which is
not so useful if .gerttyrc doesn't exist, so instead
of using that use the readfp which will open the file
and then correctly read from the file (and raise if
the file does not exist).

Change-Id: I3bf68408595e06c6ffb60632b46995c1fadbea3f