Error if `loadProvider` calls don't return successfully

[wildmolasses]

// Personally think we should error loudly here if feesSerialized not available

If loadProvider returns successfully this will never be not available (just asked railgun) so the right spot to handle this is in railgun.ts. We weren't handling the error case there originally, so worth pulling out into a separate issue? (related to #78 (comment))

Originally posted by @mds1 in #78 (comment)