Haskell style: explicit errors, visible in the type
Haskell style: explicit errors, visible in the type

While it is already standard in the Ganeti code base, explicitly
document that we do not like Exceptions and use sum types instead.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

dimara/ganeti,mbakke/ganeti,ganeti/ganeti,andir/ganeti,onponomarev/ganeti,ganeti/ganeti,dimara/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,bitemyapp/ganeti,yiannist/ganeti,apyrgio/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,andir/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,yiannist/ganeti,grnet/snf-ganeti,leshchevds/ganeti,mbakke/ganeti,yiannist/ganeti,onponomarev/ganeti,andir/ganeti,apyrgio/ganeti