Add yang.common.Error{Severity,Tag,Type} constructs

Add yang.common.Error{Severity,Tag,Type} constructs

There is a number of error-reporting constructs which are defined across
RFC4741, RFC6020, RFC6241 and RFC8040. These originate from NETCONF, but
are quite integrated in the YANG enforcement specification.

- NetconfLayer to identify the NETCONF layer involved
- ErrorSeverity to discern errors and warnings
- ErrorType to express error-types reported
- ErrorTag to extensibly identify error-tag reported

Also add proper RFC4741/RFC6241 namespace anchors and use them as
appropriate.

JIRA: YANGTOOLS-1303
Change-Id: Idbb0d7ac4a2b58f930b06d14522872a18e7037d8
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>