"Could not create TLS config: failed to read mTLS cert pair" freezes build

If the files `engflow.crt` and `engflow.key` are missing from the specified paths, `reproxy` becomes unresponsive instead of providing a clear error message.

Running `reproxystatus -watch 1s` indicated that the system status was reported as `is OK`.

Enabling verbose logging with `export RBE_v=2` produced the following logs:
```
terrym@Mac 20241128T133349.693535_65803379-8b7d-409e-b2d7-51d6e8a04288 % cat reproxy.WARNING
Log file created at: 2024/11/28 08:33:49
Running on machine: Mac
Binary: Built with gc go1.21.11 X:nocoverageredesign for darwin/arm64
Previous log:
Log line format: [IWEF]mmdd hh:mm:ss.uuuuuu threadid file:line] msg
W1128 08:33:49.733564 5489 main.go:491] Path for auxiliary metadata message descriptor file is empty.
If you want to collect backend workers' cpu/mem usage information into reproxy log, please set the --auxiliary_metadata_path flag (see instruction in api/auxiliary_metadata/readme.md).
W1128 08:33:49.735694 5489 client.go:661] Instance name was not specified.
E1128 08:33:49.735743 5489 main.go:395] Failed to initialize SDK client: failed to prepare gRPC dial options: could not create TLS config: failed to read mTLS cert pair ('/Users/terrym/engflow.crt', '/Users/terrym/engflow.key'): open /Users/terrym/engflow.crt: no such file or directory
E1128 08:33:49.744831 5489 depsscannerclient.go:65] depsscannerclient stderr:
W1128 08:33:49.744845 5489 depsscannerclient.go:590] /Users/terrym/brave-browser/src/buildtools/reclient/scandeps_server stopped with error code -1
E1128 08:33:49.744861 5489 main.go:361] Failed to initialize input processor: Failed to connect to dependency scanner service after 30 seconds: context canceled
terrym@Mac 20241128T133349.693535_65803379-8b7d-409e-b2d7-51d6e8a04288 %
```

Reproduce the failure by starting reproxy with missing engflow.crt and engflow.key files, using reproxystatus -watch 1s and RBE_v=2 as in the report. Start at the initialization errors in main.go:395, client.go:661, and depsscannerclient.go:65. Done means reproxy reports the missing mTLS certificate pair clearly and does not become unresponsive.

Written by the indexing model from the issue text.