refactor: Replace const char* to std::string

Some functions should be returning std::string instead of const char*.
This commit changes that.

src/script/script_error.cpp

@@ -5,7 +5,9 @@
 
 #include <script/script_error.h>
 
-const char* ScriptErrorString(const ScriptError serror)
+#include <string>
+
+std::string ScriptErrorString(const ScriptError serror)
 {
     switch (serror)
     {