Wrangler crashes and exits on a custom build error

### What versions & operating system are you using?

Wrangler 4.36.0

### Please provide a link to a minimal reproduction

_No response_

### Describe the Bug

Even on a simple syntax error where it does a rebuild while editing, it crashes. This is when using the custom build functionality.

Expected: don't crash, just try rebuilding again on next edit.

### Please provide any relevant error logs

_No response_

Start by reproducing the crash in Wrangler 4.36.0 using the custom build functionality, introducing a simple syntax error during editing. The fix is complete when a rebuild with that error does not crash or exit and another rebuild is attempted on the next edit.

Written by the indexing model from the issue text.