Fail better when no space in temp directory

Two separate customers now have had support issues because there was no space in the file system with the temp directory. This is what happens:
- The source control menus do not load
- The Git Web UI does not load at all
- The settings page has a misleading warning message "File not found" under the Git executable path
- There is a mysterious ILLEGAL VALUE error in the app error log

The real issue is that Embedded Git is trying to run Git commands with output redirected to a temp file and the output redirection fails when the disk is full. We can't fix this ourselves but we should make it easier to discover the problem.