Return "generic" error if a function returns an error state but does …

[hillu]

…not set error_code

Close #13

The underlying problem is documented in
hercules-team/augeas#464

[dominikh]

Are you sure that this change doesn't break other functions? We do only call error when ret is -1 or less than 0 (depending on the function), but I don't remember if any function returns a negative value while not having encountered an error.

[hillu]

I am not quite sure about New(). Other than that, the error() method is only called in actual error paths, i.e. if the corresponding Augeas C API function has returned an error value (usually -1 or otherwise negative).