Disallow loading as metamaskNetworkId

[frankiebee]

No description provided.

[frankiebee]

in a state where we aren't able to get the net id we should just fail those transactions because the txController needs some guarantee about the network

[metamaskbot]

Builds ready [7e4886b]: mascara, chrome, firefox, edge, opera

[danjm]

I think this works as an immediate solution to the bug.

However, it seems that there is a broader architectural issue with the network controller. The network state can either be an id of a network, or "loading". Then, we handle getNetworkState () returns of "loading" in various ad hoc ways throughout the controller and even in the UI.

We may be able to make the code clearer and less fragile if we store two different values in the network controller state: selectedNetwork for storing an id of the network that the user has most recently selected, and a boolean isNetworkLoading to indicate whether or not we are connected to the selectedNetwork

Something like this may also be required if we want to provide better offline support.

Of course, this is beyond the scope of this PR and should be treated as a separate issue.