[git-llvm] Check if svn is installed.

The error message that git-llvm script prints out when svn is missing
is very cryptic. I spent a fair amount of time to find what was wrong
with my environment. It looks like many newcomers also exprienced a
hard time to submit their first patches due to this error.

This patch adds a more user-friendly error message.

Differential Revision: https://reviews.llvm.org/D33458

llvm-svn: 303696