Fee monitor "Internal error" should be more specific

[seonpyoKim]

Current some error messages only send "Internal error" but it should be more specific.

[seonpyoKim]

The console error message on mainnet was like

{ error: { code: -32603, message: 'Internal error' } }
(node:9130) UnhandledPromiseRejectionWarning: #<Object>
(node:9130) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async functi
on without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:9130) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled
 will terminate the Node.js process with a non-zero exit code.

[foriequal0]

It might be an issue of internal libraries such as jayson. eslint hasn't raised any warning about floating promises.