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.

H316/h316_mt.c

@@ -551,6 +551,9 @@ switch (st) {
     case MTSE_WRP:                                      /* write protect */
         mt_err = 1;                                     /* error */
         return STOP_MTWRP;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;

I1401/i1401_mt.c

@@ -443,6 +443,9 @@ switch (st) {
 
     case MTSE_WRP:                                      /* write protect */
         return STOP_MTL;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;

I7094/i7094_mt.c

@@ -797,6 +797,9 @@ switch (st) {
 
     case MTSE_OK:                                       /* no error */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;

Interdata/id_mt.c

@@ -454,6 +454,9 @@ switch (st) {
     case MTSE_BOT:                                      /* reverse into BOT */
         uptr->UST = uptr->UST | STA_EOT;                /* set err */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }                                               /* end switch */
 
 return SCPE_OK;

NOVA/nova_mta.c

@@ -552,6 +552,9 @@ switch (st) {
         mta_upddsta (uptr, uptr->USTAT | STA_WLK | STA_RDY);
         mta_sta = mta_sta | STA_ILL;                    /* illegal operation */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;

PDP11/pdp11_tm.c

@@ -572,6 +572,9 @@ switch (st) {
     case MTSE_WRP:                                      /* write protect */
         tm_sta = tm_sta | STA_ILL;                      /* illegal operation */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;

PDP11/pdp11_tq.c

@@ -1585,6 +1585,9 @@ switch (st) {
 
     case MTSE_LEOT:
         return ST_LED;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return ST_SUC;

PDP11/pdp11_ts.c

@@ -500,6 +500,9 @@ switch (st) {
     case MTSE_WRP:                                      /* write protect */
         msgxs0 = msgxs0 | XS0_WLE | XS0_NEF;            /* can't execute */
         return (XTC (XS0_WLE | XS0_NEF, TC3));
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return 0;

PDP18B/pdp18b_mt.c

@@ -472,6 +472,9 @@ 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;

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;