tighten remote URI error handling with idiosyncratic remote names
tighten remote URI error handling with idiosyncratic remote names

Summary:
`git ls-remote` has an unusual way to indicate a URL was not
found: echoing back user input
```
$ git ls-remote --get-url does_not_exist
does_not_exist
$ echo $?
0

```

`getRemoteURI` handles checking for remotes other than 'origin', but
the error handling always matched against the string 'origin'
regardless of remote name.

Test Plan:
With a git config along the lines of:
```
[remote "my_special_name"]
        url = ssh://secure@secure.phabricator.com/diffusion/ARC/arcanist.git
        fetch = +refs/heads/*:refs/remotes/my_special_name/*
[branch "master"]
        remote = github
        merge = refs/heads/master
[remote "github"]
         # url = git@github.com:phacility/arcanist.git
         fetch = +refs/heads/*:refs/remotes/github/*
```

and running in a branch tracking `master` (github).  `arc which` would
(without this diff) show:
```
The remote URI for this working copy is "github".
```
With this diff, `arc which` correctly shows:
```
Unable to determine the remote URI for this repository.
```

When diffing against a tracking branch with a propertly configured
remote (the happy path), `arc which` still correctly identifies the
remote URI:
```
The remote URI for this working copy is
"ssh://secure@secure.phabricator.com/diffusion/ARC/arcanist.git".
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, chad, epriestley

Differential Revision: https://secure.phabricator.com/D17110

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