Fix server crashing on Lua errors

Previously, the server called FATAL_ERROR when a Lua error occured.
This caused a (mostly useless) core dump.
The server now simply throws an exception, which is caught and printed before
exiting with a non-zero return value.
This also fixes a number of instances where errors were logged multiple times.

src/exceptions.h

@@ -125,6 +125,12 @@ public:
 	PrngException(std::string s): BaseException(s) {}
 };
 
+class ModError : public BaseException {
+public:
+	ModError(const std::string &s): BaseException(s) {}
+};
+
+
 /*
 	Some "old-style" interrupts:
 */