lib : set change errmsg in sysrepo session

[Ali-aqrabawi]

setting config change error message in sysrepo session for user visibility.

for example when using sysrepo based cli:

before the change:

pc# conf t
pc(config)# lib-frr-interface 
pc(config-lib-frr-interface)# no interface virbr0
pc(config-lib-frr-interface)# commit
[ERR] User callback failed.
commit_failed: failed to commit changes!
pc(config-lib-frr-interface)#

after the change:

pc(config-lib-frr-interface)# no interface virbr0 
pc(config-lib-frr-interface)# commit 
[ERR] only inactive interfaces can be deleted
[ERR] User callback failed.
commit_failed: failed to commit changes!
pc(config-lib-frr-interface)#