Add an Errors module mirroring the Python one

As described in the module doc string, while writing this it dawned
upon me that we're mixing all errors together into a single hierarchy
(well, type on the Haskell side), which is not good. Some errors are
used purely within noded, some in the CLI frontends, etc. so these
should not be the same type; frontend functions should only be able to
raise frontend errors, not backend ones.

As to this patch itself, I've used again Template Haskell to generate
both the data type and the serialisation functions, as the initial
version, hand-written, seemed too prone to errors due to string
matching.

A small unittest for checking serialisation consistency is also added.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>