Conscript incorrectly reports "Repository not found on github" when it can't find a branch

When conscript fails to find the default "master" branch -- or the specified branch if one is specified -- then it incorrectly reports "Repository not found on github" rather than reporting that it cannot find the branch.

Example:
Type: cs sbt/sbt
Result: "Repository not found on github" because there is no "master" branch

Type: cs sbt/sbt --branch 0.13.12 # from http://www.scala-sbt.org/release/docs/Scripts.html
Result: Result: "Repository not found on github" because there is no 0.13.12 branch

Type: cs sbt/sbt --branch 0.13
Result: works (at the time of this writing)

This was very confusing as a complete newbie trying to figure out what was going wrong until I realized it simply wasn't findng the branch.

Reproduce the issue with `cs sbt/sbt`, `--branch 0.13.12`, and `--branch 0.13` to confirm the different branch outcomes. Trace the branch lookup and error-reporting path; done means a missing default or specified branch reports that the branch cannot be found instead of claiming the repository is missing.

Written by the indexing model from the issue text.