Clear errstack after successful auth'n. #7774
Clear errstack after successful auth'n. #7774

After we have successfully authenticated, we should clear out the
error state object.  Otherwise, any prior authentication failure
is included in the error stack resulting in a confusing error message
for authorization failures.

Before, on a failed `condor_submit`:
```
ERROR: Failed to connect to local queue manager
SECMAN:cf1cc2470562cb70d989a4e1426fc76eeb516957@07a87ab21a6b using method FS.
AUTHENTICATE:1004:Failed to authenticate using KERBEROS
```

(Here, I had included `KERBEROS` as the first authentication protocol even though it
is guaranteed to fail in my setup.)

After:
```
ERROR: Failed to connect to local queue manager
SECMAN:cf1cc2470562cb70d989a4e1426fc76eeb516957@07a87ab21a6b using method FS.
```

htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor