Ensure all error prefixes in the "database/sql" package start with
"sql: ". Do not prefix errors for type conversions because they
are always embedded in another error message with a specific
context.

Fixes #25251

Change-Id: I349d9804f3bfda4eeb755b32b508ec5992c28e07
Reviewed-on: https://go-review.googlesource.com/111637Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>