loader command interpreter should reset command_errmsg

[tsoome]

Summary

The error message from previous command is repeated in case the forth
word is defined as builtin.

Illumos issue: https://www.illumos.org/issues/7405

Test Plan

I did use the following to reproduce the issue in fbsd:
OK include /boot/pcibios.4th
OK lszfs <-- this will result with error message being printed
OK pci-device-count 1

last command will do it's output *and* will print error message from
lszfs.

[allanjude]

Approved for commit