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/script/common/c_types.h

@@ -52,10 +52,10 @@ public:
 	}
 };
 
-class LuaError : public ServerError
+class LuaError : public ModError
 {
 public:
-	LuaError(const std::string &s) : ServerError(s) {}
+	LuaError(const std::string &s) : ModError(s) {}
 };