REST: Use status 400 for method arg problems

Use 400, which is a client error, for errors when
deciphering method call arguments as opposed to
a server error, 500.  The robot testcases expect this
and it also makes sense since the client must have
passed in a bad argument.

Only the error that results from a method call failing
will be 500.

Resolves openbmc/bmcweb#25

Change-Id: I9504905992fee3bc0119ff7cae8512eba899daa4
Signed-off-by: Matt Spinler <spinler@us.ibm.com>