Add mechanism to send error messages to helper.

Adds a new err_msg function which is used to:

-echo feedback to the deploy ramdisk console
-keep track of the first error message we hit so
 that we can send it along to the baremetal-deploy-helper.

Also, updates our wget request back to baremetal-deploy-helper
to include the first the first error message (if any) as
the 'e' parameter.

The err_msg uses a new simple safe_url_encode function to ensure
we don't send invalid characters in our HTTP post requests.

Change-Id: I5a623a6f66cde8d81ff1e75800dc2953ca2703a8
Merge "Add mechanism to send error messages to helper."