fix: fail sign-dlls action when no matching DLLs are found

[kinyoklion]

Summary

The sign-dlls action silently succeeds when dll_name doesn't match any files — the find returns nothing and the for loop body never executes. This allowed dotnet-eventsource to ship 3 releases without Authenticode signing due to a typo in the dll_name input (see launchdarkly/dotnet-eventsource#121).

Added a guard that checks find results before the loop and fails with ::error:: if no DLLs are found. All current callers provide correct names, so no existing workflows will break.

Link to Devin session: https://app.devin.ai/sessions/226ed88b8e8a4434920f2d7e3a49ee61
Requested by: @kinyoklion

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