Add warning if GAZEBO_RESOURCE_PATH may not be set correctly (#2991)

* Add error message to RT shader
* Improve error message for loading worlds/empty.world

Signed-off-by: Audrow Nash <audrow@hey.com>

gazebo/Server.cc

@@ -350,7 +350,13 @@ bool Server::ParseArgs(int _argc, char **_argv)
     {
       gzwarn << "Falling back on worlds/empty.world\n";
       if (!this->LoadFile("worlds/empty.world", physics))
+      {
+        gzerr << "worlds/empty.world could not be opened, "
+              << "probably because it was not found. "
+              << "Your GAZEBO_RESOURCE_PATH is probably improperly set. "
+              << "Have you sourced <prefix>/share/gazebo/setup.sh?\n";
         return false;
+      }
     }
 
     if (this->dataPtr->vm.count("profile"))

gazebo/rendering/RTShaderSystem.cc

@@ -469,7 +469,9 @@ bool RTShaderSystem::GetPaths(std::string &coreLibsPath, std::string &cachePath)
   // Core shader lib not found -> shader generating will fail.
   if (coreLibsPath.empty())
   {
-    gzerr << "Unable to find shader lib. Shader generating will fail.";
+    gzerr << "Unable to find shader lib. Shader generating will fail. "
+          << "Your GAZEBO_RESOURCE_PATH is probably improperly set. "
+          << "Have you sourced <prefix>/share/gazebo/setup.sh?\n";
     return false;
   }