vty: change output of errors from mgmtd

vty: change output of errors from mgmtd

Make errors look the same way as in regular non-mgmtd vty. We don't need
to show information about some internal request names.

Before:
```
ERROR: SET_CONFIG request failed, Error: YANG error(s):
 Path: Data location "/frr-affinity-map:lib/affinity-maps/affinity-map[name='a']".
 Error: Unique data leaf(s) "value" not satisfied in "/frr-affinity-map:lib/affinity-maps/affinity-map[name='b']" and "/frr-affinity-map:lib/affinity-maps/affinity-map[name='a']".
```

After:
```
% Configuration failed.

YANG error(s):
 Path: Data location "/frr-affinity-map:lib/affinity-maps/affinity-map[name='b']".
 Error: Unique data leaf(s) "value" not satisfied in "/frr-affinity-map:lib/affinity-maps/affinity-map[name='a']" and "/frr-affinity-map:lib/affinity-maps/affinity-map[name='b']".
```

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>