Remove redundant error display

The only caller to `getTemplate` already handles displaying an error, so the
check here was redundant.

Test Plan:
Crossed fingers
Use a single RemoteRevision per editor window

Parameterize `url()` by the revision instead having it be fixed at creation
time to allow all `BlameLineView` components to share the same instance.

Test Plan:
Toggled blame view and it still works. Also, specs still pass.
Validate custom URL template string once, not once per line

Fixes #26

Test Plan:
Made `getTemplate` return null, then triggered blame. Before this change one
error would appear for every line. After this change, a single error appears.
Merge pull request #28 from dmnd/error-deluge

Fix error deluge