Do not bail out if 'load file' command has unavailable file,

just post error message and continue
A fix to https://sourceforge.net/p/ngspice/discussion/127605/thread/986c04f22b/
by Clyde

src/frontend/rawfile.c

@@ -304,7 +304,7 @@ raw_read(char *name) {
 
     if ((fp = fopen(name, "rb")) == NULL) {
         perror(name);
-        controlled_exit(EXIT_FAILURE);
+        return NULL;
     }
 
     /* Since we call cp_evloop() from here, we have to do this junk. */