proj: add a hint in error message about using cct when relevant (#4016)

Fixes #4013.

src/apps/proj.cpp

@@ -640,7 +640,7 @@ int main(int argc, char **argv) {
 
     if (!proj_angular_input(Proj, PJ_FWD)) {
         emess(3, "can't initialize operations that take non-angular input "
-                 "coordinates");
+                 "coordinates. Try cct.");
         exit(0);
     }