Hamming: Make error checking identical to other tests

[jonasbb]

I noticed this after @petertseng mentioned the error handling in #147.
The Hamming exercise performs error checking with comparison to Err(.). Most tests (Nucleotide Codons, Queen Attack, Wordy) only check with is_err(). The exceptions are Circular Buffer and Forth, but they use more complex error reporting with different conditions and provide an Error enum with the correct return values.

Because there is only one error conditions I suggest to remove the fixed value and only check with is_err().

[IanWhitney]

Thanks. I like it.