db: reconsider commit error handling

[RaduBerinde]

commitWrite currently panics on synchronous WriteRecord errors:

pebble/db.go

Line 951 in f140dd5

panic(err)

The only way this can error is if w.err is set here:

pebble/record/log_writer.go

Line 975 in b189a4e

if w.err != nil {

w.err is set on close and when the flusher is in error state when we queue a block:

pebble/record/log_writer.go

Line 863 in b189a4e

w.err = w.flusher.err

However, the flusher retries, so it's unclear that we should be exposing this error at all.

The error handling when commitWrite (as the commitEnv.write callback) returns an error is incomplete, at least when the error is ErrInvalidBatch.

See #5984

Jira issue: PEBBLE-1429