build: better error if fetching AMP validator fails (#73851)
if this `fetch` fails it's kinda annoying to find, because it just gets
printed as
```
│ [17:05:40] precompile failed because fetch failed
```
this change should at least make it easier to find the cause by
searching for the error message:
```
│ [17:20:27] precompile failed because Failed to fetch AMP validator
```
