make permissions fail in rez-release more obvious

Currently if you attempt to rez-release to a directory that you cannot write to, you tend to get the not-obvious error:

rez.vendor.lockfile.LockFailed: failed to create /somserver/packages/va1ws5219-62801700.11865-1188444279736822984

Instead, you should get a nicer error, such as:

It appears you do not have write permissions on /someserver/packages. The release was aborted.

Start at the rez-release path that reports rez.vendor.lockfile.LockFailed when creating the release directory. Trace how a failed write or lock creation is surfaced, then verify that an unwritable destination produces a clear permissions error naming the target directory and that the release is aborted.

Written by the indexing model from the issue text.