patch 9.0.1576: users may not know what to do with an internal error

Problem:    Users may not know what to do with an internal error.
Solution:   Add a translated message with instructions.
diff --git a/src/regexp_nfa.c b/src/regexp_nfa.c
index b7bfc86..72e5e25 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -1401,7 +1401,7 @@
 		    rc_did_emsg = TRUE;
 		    return FAIL;
 		}
-		siemsg("INTERNAL: Unknown character class char: %d", c);
+		siemsg("Unknown character class char: %d", c);
 		return FAIL;
 	    }
 
@@ -6905,7 +6905,7 @@
 
 #ifdef DEBUG
 		if (c < 0)
-		    siemsg("INTERNAL: Negative state char: %ld", (long)c);
+		    siemsg("Negative state char: %ld", (long)c);
 #endif
 		result = (c == curc);
 
@@ -7196,9 +7196,9 @@
     if (f != NULL)
     {
 	fprintf(f, "\n\n\t=======================================================\n");
-#ifdef DEBUG
+# ifdef DEBUG
 	fprintf(f, "\tRegexp is \"%s\"\n", nfa_regengine.expr);
-#endif
+# endif
 	fprintf(f, "\tInput text is \"%s\" \n", rex.input);
 	fprintf(f, "\t=======================================================\n\n");
 	nfa_print_state(f, start);