This removes a source of complexity and indirection that makes it harder to
understand path checking code. Path checks will be simplified in upcoming
commits.
There is no change in behavior in this commit other than a slightly more
descriptive error message in `loadwallet` if the default "" wallet can't be
found. (The error message is improved more in upcoming commit "wallet: Remove
path checking code from loadwallet RPC".)
Instead of removing the database transaction logs and retrying the
wallet loading, just return an error message to the user. Additionally,
specifically for DB_RUNRECOVERY, notify the user that this could be due
to different BDB versions. This error is pretty much only caused by
compiling with a newer version of BDB and then trying to open the wallet
with a version compiled with an older version of BDB.