Show (short) list of possible enum values when an invalid value is encountered (resolves #1318)

All enum errors have (hopefully) been changed to a luax_enumerror, which has a
fixed error message. If additionally a list of valid options is passed, it
lists that in the error message. For every enum error with few options I've
implemented this using a getConstants call.

This solution has been designed specifically to reduce the number of template
instantiations (as I've been told that was a concern). All new template code
happens in places where there already was an instantiation of the relevant
StringMap. Of course there is still std::vector<std::string>...

--HG--
branch : minor

src/modules/filesystem/wrap_File.cpp

@@ -77,7 +77,7 @@ int w_File_open(lua_State *L)
 	File::Mode mode;
 
 	if (!File::getConstant(str, mode))
-		return luaL_error(L, "Incorrect file open mode: %s", str);
+		return luax_enumerror(L, "file open mode", File::getConstants(mode), str);
 
 	try
 	{
@@ -353,7 +353,7 @@ int w_File_setBuffer(lua_State *L)
 
 	File::BufferMode bufmode;
 	if (!File::getConstant(str, bufmode))
-		return luaL_error(L, "Incorrect file buffer mode: %s", str);
+		return luax_enumerror(L, "file buffer mode", File::getConstants(bufmode), str);
 
 	bool success = false;
 	try