providers: common: make exception code/message standard
    
    This makes sure that any ProviderException can have an error
    code for passing on to the client, and adds default error
    codes for subclasses.
    
    Fixes: #212
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Rob Crittenden <rcritten@redhat.com>