Return to the main menu if a shader compilation fails (#14256)

Before this change, if the shaders are broken, only an error message is shown and the player enters the world nonetheless, where he/she sees broken graphics.

src/exceptions.h

@@ -92,6 +92,11 @@ class PrngException : public BaseException {
 	PrngException(const std::string &s): BaseException(s) {}
 };
 
+class ShaderException : public BaseException {
+public:
+	ShaderException(const std::string &s): BaseException(s) {}
+};
+
 class ModError : public BaseException {
 public:
 	ModError(const std::string &s): BaseException(s) {}