docs($httpBackend): add catch() block to example

[xfg]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Docs update.

What is the current behavior? (You can also link to an open issue here)
MyController example does not pass through the test it should fail authentication

What is the new behavior (if this is a feature change)?
MyController example passes through the test it should fail authentication.

Does this PR introduce a breaking change?
No.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format

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[xfg]

I signed it!

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