Fix failed test 'exception gives good detail'

Fixes rizen/Facebook-Graph/issues#49 - it looks like Facebook just started appending "Please read the Graph API documentation at https://developers.facebook.com/docs/graph-api" to the error message. Since it's not the relevant part of the error message, ignoring it should actually make the test more robust.

With the patch:
```
# perl -I lib/ t/03_public_query.t
1..13
ok 1 - use Facebook::Graph;
ok 2 - An object of class 'Facebook::Graph' isa 'Facebook::Graph'
ok 3 - An object of class 'Facebook::Graph::Query' isa 'Facebook::Graph::Query'
ok 4 - seems to generate uris correctly
ok 5 - An object of class 'Facebook::Graph::Response' isa 'Facebook::Graph::Response'
ok 6 - got a hash ref back
ok 7 - got sarah
ok 8 - know her name
ok 9 - only fetched the things i asked for
ok 10 - she is a user
ok 11 - exception inherits http status code
ok 12 - exception gives good detail
ok 13 - can directly fetch a facebook graph url
```

Thank you for considering this pull request!