cli: Remove code reference from error messages

We use Lua's error/assert to raise errors from commands. Lua makes it a bit
more informative and prefixes with filename and line number. We don't need
this, but we still want to use error/assert.

This detail (file and line number) is added internally by Lua (using the method
luaL_where()), so this patch does a string lookup to remove it. It does not do
it for internal CLI errors (in the C implementation) because we need these
details for these types of errors.

Fixes #414.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>