Use the msg function to report errors wherever possible.

Use the msg function to report errors wherever possible.

Previously, some errors were reported using the GNU "error" function from error.h
rather than the PSPP msg function.  This wasn't too bad for the terminal user interace,
but gui users could miss important messages (since they never see stderr).

The reason for not using msg in some instances, was twofold: 1) Some
errors might occur before the messaging system had been initialised. 2) If an
error occured whilst reporting an error, an infinite loop would occur.

This change overcomes these problems by detecting re-entrancy, and uninitialised
messaging system, and falling back to fwrite if either of these conditions occur.