invalid config path is silently ignored

Repro:

1. `tsup --config YOURSTUPIDFACE`

Expected: an error that tsup couldn't find YOURSTUPIDFACE
Actual: tsup builds with defaults and no warnings/errors

I had a typo in my config filename and such an error would have saved me much time.

Thank you!

Start by tracing how the tsup CLI handles the --config argument in the reproduced command. Check the behavior for a nonexistent config path and confirm that a clear error is reported instead of silently using defaults; verify the existing valid-config behavior remains unchanged.

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