Fixed invalid checksum error response from sandbox
Fixed invalid checksum error response from sandbox

chef_wm_malformed_message received a clause for handling fun_match
errors. As a result, it passes the error message provided by the
fun_match, rather than the generic, "Field 'checksums' invalid".

Sandbox endpoint uses a fun_match, and the sandbox tests were updated to
reflect this change.

chef/chef-pedant,chef/chef-pedant,chef/chef-pedant