GN: update error message for invalid input_conversion.

The current message is off-putting to read every time I get
input_conversion wrong.

BUG=

Review-Url: https://codereview.chromium.org/2186683003
Cr-Original-Commit-Position: refs/heads/master@{#409008}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1c41ccaca266ec1272e5430a9fdc7145af043d20
diff --git a/tools/gn/input_conversion.cc b/tools/gn/input_conversion.cc
index 043737a..c14f4ae 100644
--- a/tools/gn/input_conversion.cc
+++ b/tools/gn/input_conversion.cc
@@ -140,7 +140,7 @@
     return ParseValueOrScope(settings, input, PARSE_SCOPE, origin, err);
 
   *err = Err(original_input_conversion, "Not a valid input_conversion.",
-             "Have you considered a career in retail?");
+             "Run gn help input_conversion to see your options.");
   return Value();
 }