Export of internal Abseil changes

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667d6502169b645de44d5aa79ff7524c329ef136 by Abseil Team <absl-team@google.com>:

Improve the error messages for absl::Substitute to clarify that an unescaped $ symbol is an error.

The existing functionality enforces that unescaped $ results in a compile-time error. However, the current error messages do not mention this case and make it easier to miss while looking for an incorrect numerical substitution argument.

PiperOrigin-RevId: 390242485
GitOrigin-RevId: 667d6502169b645de44d5aa79ff7524c329ef136
Change-Id: Iad1f1c18ab9686be11d40772e3d51dd233cfc9bc