Check for the existence of git before running commands

gclient_scm.py

Index: gclient_scm.py
diff --git a/gclient_scm.py b/gclient_scm.py
index 6986bb7eb4bbdea5282e529427cf6925e669dc8d..93f69974b83db0e4a10c0e9340c5ca2e4679734d 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -286,8 +286,13 @@ class GitWrapper(SCMWrapper, scm.GIT):
       cwd = self.checkout_path
     cmd = [self.COMMAND]
     cmd.extend(args)
-    sp = subprocess.Popen(cmd, cwd=cwd, stdout=stdout)
-    output = sp.communicate()[0]
+    logging.debug(cmd)
+    try:
+      sp = subprocess.Popen(cmd, cwd=cwd, stdout=stdout)
+      output = sp.communicate()[0]
+    except OSError:
+      raise gclient_utils.Error("git command '%s' failed to run." %
+              ' '.join(cmd) + "\nCheck that you have git installed.")
     if checkrc and sp.returncode:
       raise gclient_utils.Error('git command %s returned %d' %
                                 (args[0], sp.returncode))