Make skydb start url error out nicely

sky/tools/skydb

Index: sky/tools/skydb
diff --git a/sky/tools/skydb b/sky/tools/skydb
index eca44490be3c8cd6baf4c43572174047600dfce1..02c92412bb7335cc860e284fd29e57b463016eab 100755
--- a/sky/tools/skydb
+++ b/sky/tools/skydb
@@ -141,10 +141,15 @@ class SkyDebugger(object):
     def start_command(self, args):
         # FIXME: Lame that we use self for a command-specific variable.
         self.paths = self._create_paths_for_build_dir(args.build_dir)
-
         self.stop_command(None) # Quit any existing process.
         self.pids = {} # Clear out our pid file.
 
+        if not os.path.exists(self.paths.mojo_shell_path):
+            print "mojo_shell not found in build_dir '%s'" % args.build_dir
+            print "Are you sure you sure that's a valid build_dir location?"
+            print "See skydb start --help for more info"
+            sys.exit(2)
+
         # FIXME: This is probably not the right way to compute is_android
         # from the build directory?
         gn_args = gn_args_from_build_dir(args.build_dir)