(PUP-3222) Rescue StandardError instead of SystemCallError

Commit c752a9ceea was supposed to rescue StandardErrors not
SystemCallErrors so that we do not break in the same way again
if the win32-service gem decides to raise a different exception.

This commit restores rescuing of StandardErrors.
(maint) Generate unique account names

Previously, the specs could fail if the same test was executed in
parallel, since the test assumed the account didn't exist.

This commit makes the names unique. The maximum length for local
user and groups accounts in 2003 is 20 and 256 characters[1],
respectively, and we remain below that limit in both cases.

[1] http://technet.microsoft.com/en-us/library/cc783323.aspx