errors: add support for generic errors

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
pom.xml: update ahc to 1.7.20

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
client: improve errors handling

Exceptions were sometimes swallowed in the callback.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
client: fix test failure

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
model: add description to Transaction

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>