No warning when one of multiple entry points is not valid

When using a single entry point that cannot be found, tsup errors and informs the user. But when using multiple entry points, one of which is invalid, the invalid entry point will quietly be ignored.

This can cause unexpected build behaviour. Users are also likely to stick with invalid configs for longer, as they don't realise there is something wrong.

## Expected Bahaviour
Fail compilation if any of the entry points are invalid & inform the user as to what happened

The issue concerns tsup's handling of multiple entry points; start by reproducing a build with one valid and one missing entry point. Trace the entry-point validation path and the existing single-entry error behavior. Done means compilation fails and identifies the invalid entry point.

Written by the indexing model from the issue text.