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.

使用 `cs sbt/sbt`、`--branch 0.13.12` 和 `--branch 0.13` 重现该问题，以确认不同分支的结果。跟踪分支查找和错误报告路径；当缺失的默认分支或指定分支报告找不到该分支，而不是声称仓库缺失时，即表示完成。

由索引模型根据 Issue 内容生成。