rpc.py: expliclty print the exception's message

This worked fine as-is with Python 3.  But when
trying to run rpc.py with Python 2, it was noted
that simply doing print(ex) didn't print the message,
it needs to be print(ex.message).

Note that the similar code for printing the exception
message when running an RPC script was already doing
print(ex.message), so this change also makes the
code more consistent.

We aren't supporting or testing rpc.py with Python 2,
and there are some fairly significant mods required
to get it to work, but this small change at least
reduces that effort for those needing to implement
it and maintain it themselves.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I0a6c94889dc631ef33835433f20f35ddc738aafb

Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/476633


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Alexey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Community-CI: Broadcom SPDK FC-NVMe CI <spdk-ci.pdl@broadcom.com>