Succeed if $sha does not exist (and reduce noise).

I don't know if anyone else experiences this, but we're receiving email notifications for every IDE validation failure. A lot of them are spurious (see blow, probably force-pushing on the PR before the validation script got the chance to start). This is my attempt to reduce noise.

```
From github.com:scala/scala
 * branch            master     -> FETCH_HEAD
++ [[ -f <https://scala-webapps.epfl.ch/jenkins/job/pr-scala-integrate-ide/ws/mergetarget> ]]
++ git checkout --detach -f 8848f241616627b0c5beca38a5107c4eca3e10fd
HEAD is now at 8848f24... Merge pull request #3007 from densh/pull/fresh-name-and-package-support
++ git clean -fxd
++ git merge 50f30dcf52a6fb8f1db4487e524ef0c78a6eef64
fatal: '50f30dcf52a6fb8f1db4487e524ef0c78a6eef64' does not point to a commit
Build step 'Execute shell' marked build as failure
```