Gracefully handle errors better if xml command fails

[ktbyers]

The below would just come back as a generic Netmiko error intead of raising the underlying error that happened from the xml command.

Would be better if the xml command that resulted in an ERROR raised the underlying error message.

RP/0/RP0/CPU0:ios-xr#
RP/0/RP0/CPU0:ios-xr#terminal length 0
Fri Oct 12 19:56:36.548 UTC
RP/0/RP0/CPU0:ios-xr#terminal width 511
Fri Oct 12 19:56:36.774 UTC
RP/0/RP0/CPU0:ios-xr#
RP/0/RP0/CPU0:ios-xr#xml
Fri Oct 12 19:56:37.485 UTC
ERROR: 0xa3679c00 'XML Service Library' detected the 'fatal' condition 'The router is low on memory. Please retry the request after memory consumption has gone down.'
RP/0/RP0/CPU0:ios-xr#<?
                     ^
% Invalid input detected at '^' marker.
RP/0/RP0/CPU0:ios-xr#<xml version="1.0" encoding="UTF-8"?
                     ^
% Invalid input detected at '^' marker.
RP/0/RP0/CPU0:ios-xr#<xml version="1.0" encoding="UTF-8"><Request MajorVersion="1" MinorVersion="0"><Get><Operational><SystemTime/><PlatformInventory/>        </Operational></Get></Request>