Add a warning if a fetch method never resolves or rejects

[ElliotNB]

If a fetch method is improperly coded and does not eventually resolve or reject, then the Promise.all in TXMBase._fetch is never invoked and it can lead to issues with refreshing and this.pendingFetchCount will never equal 0 which means that this.isReady will always return false.

[ElliotNB]

Resolved.