[perf] Don't allocate argument mismatch string when errors gagged

[ntrel]

@Geod24 This should fix the high memory use found here (Note: #8232 already helped this somewhat).

[dlang-bot]

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[Geod24]

@ntrel : Thanks for taking care of this! Glad you could come up with a solution. I didn't have much time to look into D since DConf, but I'll try to build my project from stable this weekend and see if memory usage has returned to normal.