Add perror() to fopen: more info for user.

src/frontend/device.c

@@ -1542,6 +1542,7 @@ com_alter_mod(wordlist *wl)
 
     if (modfile == NULL) {
         fprintf(cp_err, "Warning: Could not open file %s, altermod ignored\n", filename);
+        perror("    Cause: ");
         tfree(input);
         tfree(filename);
         return;

src/frontend/inpcom.c

@@ -583,6 +583,7 @@ static struct library *read_a_lib(const char *y, const char *dir_name)
 
         if (!newfp) {
             fprintf(cp_err, "Error: Could not open library file %s\n", y);
+            perror("    Cause: ");
             return NULL;
         }
 
@@ -1564,6 +1565,7 @@ static struct inp_read_t inp_read(FILE* fp, int call_depth, const char* dir_name
                         else
                             fprintf(cp_err, "    While reading %s\n", y_resolved);
                     }
+                    perror("    Cause: ");
                     tfree(buffer); /* allocated by readline() above */
                     controlled_exit(EXIT_FAILURE);
                 }

src/frontend/measure.c

@@ -255,8 +255,10 @@ do_measure(
 
     if (cp_getvar("measoutfile", CP_STRING, out_file, sizeof(out_file))) {
         measout = fopen(out_file, "w");
-        if (!measout)
+        if (!measout) {
             fprintf(stderr, " Warning: Could not open file %s\n", out_file);
+            perror("    Cause: ");
+        }
     }
 
     /* Evaluating the linked list of .meas cards, assembled from the input deck