[prpc] Warn about invalid headers
[prpc] Warn about invalid headers

Currently we log an exception if a client passed an invalid header.
This generates an error report to service maintainers. This is unnecessary.

Use warning level, not exception level for logging this.

Change-Id: If576fbcd917af404609aa5d8575b428f7040fbc5
Reviewed-on: https://chromium-review.googlesource.com/1086121
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py