Allow return code of '1' from 'svn propget'

SVN 1.9 has apparently decided to break the "API" of the 'svn propget' command
by making a return value of '1' indicate success. This change adds that return
code to the allowed errors for the command.

Testing done:
I don't have SVN 1.9, but I made sure that propget still worked correctly on
1.7.x.

Fixes bug 3566.

Reviewed at https://reviews.reviewboard.org/r/6325/
Merge branch 'release-0.6.x'