uReport: add more explanatory message when upload fails

- closes #135

Signed-off-by: Jakub Filak <jfilak@redhat.com>
uReport: detect missing type field at client side

Old output:
Server side error: 'Validation failed: missing mandatory element 'type''

New output:
An unsupported value 'anaconda' in problem element 'analyzer'
Can't create an uReport without 'type'
Not uploading an empty uReport
Failed on submitting the problem

- closes #136

Signed-off-by: Jakub Filak <jfilak@redhat.com>
uReport: do not show URL twice in error output