Fix a bug in acquireImage exception handling.

Previously, a non-zero status to SANE_NET_START would cause JFreeSane to throw
an exception as soon as the bad status was read from the input stream. The
remaining bytes of SANE's response to the SANE_NET_START would remain on the
stream, leaving JFreeSane in the wrong state to begin handling subsequent
requests.

This was particularly evident in situations where you expect a non-zero scan
result, e.g. in ADF scanning (where the last page will return a STATUS_NO_DOCS
error).
Merge pull request #15 from sjamesr/fix_acquire_image

Fix a bug in acquireImage exception handling.