Make the checkout_pgo_profiles error message more explicit

Change-Id: Ife0bbd6baa9deedac77a777ddc672d723dd276c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255121Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780484}

tools/update_pgo_profiles.py

@@ -105,8 +105,20 @@ def _get_profile_path(args):
   if not os.path.isfile(profile_path):
     raise RuntimeError(
         'requested profile "%s" doesn\'t exist, please make sure '
-        '"checkout_pgo_profiles" is set to true in the "custom_vars" section '
-        'of your .gclient file and then run "gclient runhooks" to download it' %
+        '"checkout_pgo_profiles" is set to True in the "custom_vars" section '
+        'of your .gclient file, e.g.: \n'
+        'solutions = [ \n'
+        '  { \n'
+        '    "name": "src", \n'
+        '    # ...  \n'
+        '    "custom_vars": { \n'
+        '      "checkout_pgo_profiles": True, \n'
+        '    }, \n'
+        '  }, \n'
+        '], \n'
+        'and then run "gclient runhooks" to download it. You can also simply '
+        'disable the PGO optimizations by setting |chrome_pgo_phase = 0| in '
+        'your GN arguments.'%
         profile_path)
 
   os.utime(profile_path, None)