LoadAccountMergeAmount does not check for horizon error

[mlsteele]

Client.LoadAccountMergeAmount makes a request to horizon but does not check for an error. As a result if there is a horizon error it will likely always return this error, instead of the horizon error.

Could not find `account_credited` effect in `account_merge` operation effects`.

It should probably use something like decodeResponse to unmarshal the horizon response. Although I don't understand why that method accepts http status [200, 300) instead of just 200.

Here is a hacked up standalone example of the gist of this bug.

[robertDurst]

Good catch! Checking this out

[robertDurst]

Opening a pr -- yes, this should use decodeResponse since decodeResponse does more than just decode, it checks status and then if it is unsuccessful, returns the error.