In the New Review Request page, if there's any sort of issue loading the

list of branches or commits from a repository, the result is a blank

list of branches or commits with no error message, and an error e-mail

sent to administrators. This is not very helpful.
Now when there's an error loading any data from the server, an error

box will appear saying there was an issue loading from the server,

and showing the server-provided error message. The user will be given

the option to try the request again.
For this to work, RepositoryBranchesCollection had to be made a subclass

of BaseCollection, and BaseCollection had to be given support for

parsing and storing errors on the xhr, similar to what BaseResource

does. This code has been pulled out into a common function that both

classes now use.
The RepositoryBranches and RepositoryCommits resources now return a

proper error code when encountering a SCMError. We're reusing the "Error

fetching information from this repository" error, with a custom message

(provided by the SCMError).

Tested without any errors from the repository.
Simulated branch failure. Clicked a repository and saw an error message,

with the branch selector gone. I then clicked "Try Again," and got the

same result. Then I turned off the simulated error, tried again, and got

the full branches and commits list.
Simulated commits failure. Clicked a repository and saw the branches, and

an error message. I then clicked "Try Again," and got the same result. Then

I turned off the simulated error, tried again, and got the full commits list.
Unit tests pass.