Add verbosity levels for warnings and errors.

lib/src/git.dart

Index: lib/src/git.dart
diff --git a/lib/src/git.dart b/lib/src/git.dart
index f2c2c226416453c4ea502e343738c683a7661d71..087a995c70440671e2325ccc0b5ebb48e0ce96ac 100644
--- a/lib/src/git.dart
+++ b/lib/src/git.dart
@@ -117,7 +117,7 @@ bool _tryGitCommand(String command) {
   } on ProcessException catch (error, stackTrace) {
     var chain = new Chain.forTrace(stackTrace);
     // If the process failed, they probably don't have it.
-    log.message('Git command is not "$command": $error\n$chain');
+    log.error('Git command is not "$command": $error\n$chain');
     return false;
   }
 }