Various Magtape Devices: Handle Coverity report of tape error status cases

Coverity flagged one magtape simulator for having fewer case breakouts
than the range of MTSE_x errors, after they were expanded for
MTSE_LEOT (listed as "V4 compatibility") and MTSE_RUNAWAY (for
Dave Bryan's implementation of erase and erase gap).

According to Dave, it turns out that MTSE_RUNAWAY can only be
returned if the magtape simulator supports and implements a density
specification and the ERASE function. The simulators Bob Supnik wrote
don't meet that criteria, so the expanded errors can't be returned.

Simply add a "default" case to the map logic returning SCPE_IERR for
unexpected cases.

PDP8/pdp8_mt.c

@@ -590,6 +590,9 @@ switch (st) {
     case MTSE_WRP:                                      /* write protect */
         mt_sta = mt_sta | STA_ILL | STA_ERR;            /* illegal operation */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;