Port yuzu-emu/yuzu#4528: "common: Make use of [[nodiscard]] where applicable" (#5535)

Co-authored-by: LC <712067+lioncash@users.noreply.github.com>

src/common/common_funcs.h

@@ -57,4 +57,4 @@ __declspec(dllimport) void __stdcall DebugBreak(void);
 // Call directly after the command or use the error num.
 // This function might change the error code.
 // Defined in Misc.cpp.
-std::string GetLastErrorMsg();
+[[nodiscard]] std::string GetLastErrorMsg();