fix: Don't persist cache files that error while writing

[loewenheim]

This adds an early return in memory.rs so that if an InternalError is returned by a cache computation¹, we don't attempt to write the error and don't persist the temp file. This kills two birds with one stone:

1. It fixes the "Internal errors should never be written out" issue we see in Sentry.
2. More importantly, if a computation fails with an InternalError because of lack of disk space, we discard the resulting file immediately instead of preserving its (likely useless) contents.

Moreover, I also added some extra validation after symcache/ppdbcache/sourcemapcache conversion. Now, if somehow the write goes through but the file is invalid, we also return an InternalError and it will be deleted. This costs us nothing because parsing these files is essentially free. I thought about doing the same for cficache files, but there the tradeoff is less clear—parsing them takes significant resources, for a case we don't know ever occurs.

¹ Caused by an io::Error, for instance.