cmd/preprofile: clean up error handling

cmd/preprofile: clean up error handling

This CL adjusts error handling to be a bit more idiomatic. The
processing function returns errors, leaving main to log and exit on
error.

This CL contains no functional changes.

For #58102.

Change-Id: I9074127cc675e177d046474b7f01fbc37d0bd4c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/569335
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>