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