Update error message in color check.

Remind user if 'ANDROID_BUILD_TOP' hasn't been set

Bug: 65850237
Test: In terminal without android environment, try
      to run 'repo upload'

Change-Id: I7cbd903d3b2ac08ddbd3ae6912e7d1e0bc76d2ca
diff --git a/checkcolor.py b/checkcolor.py
index 9e62d3d..204c7e5 100755
--- a/checkcolor.py
+++ b/checkcolor.py
@@ -72,6 +72,9 @@
   except OSError as e:
     if e.errno == errno.ENOENT:
       print 'Error in color lint check'
+      if not os.environ.get('ANDROID_BUILD_TOP'):
+        print 'Try setting up your environment first:'
+        print '    source build/envsetup.sh && lunch <target>'
       sys.exit(1)
 
   return (exitcode, stdout)