log messages for web/pam authentication failures for support troubleshoo...

...ting

DEMO - failure when user is not part of admin group:
```
E1023 18:57:56.317454 12075 auth.go:70] PAM result for user:huey group:sudo was 4: pam: invalid admin group
I1023 18:57:56.317510 12075 session.go:161] Attempting to validate user huey against the control center api
E1023 18:57:56.319371 12075 session.go:172] Unable to validate credentials No such entity {kind:user, id:huey}
2014/10/23 18:57:56 401 20.488801ms POST /login
```

DEMO2 - failure when user supplies wrong password:
```
cauth: pam_authenticate returned 7: Authentication failure
E1023 19:04:22.655161 12075 auth.go:70] PAM result for user:plu group:sudo was 2: pam: authentication failed
I1023 19:04:22.655215 12075 session.go:161] Attempting to validate user plu against the control center api
E1023 19:04:22.657198 12075 session.go:172] Unable to validate credentials No such entity {kind:user, id:plu}
```