Improve SchemaInferenceStack error reporting

Improve SchemaInferenceStack error reporting

Include a description of parent's identity when we fail to locate
its child. This makes it easier to understand what is going on in most
failure situations.

The message now ends with something like
- not present in schema parent (namespace)parentName
- not present in grouping (namespace)groupingName
- not present in module (namespace)moduleName
or similar.

Also expand the test suite to cover negative scenarios around
enterChoice().

JIRA: YANGTOOLS-1336
Change-Id: I8a642dcd0fe5705f4c33c1a3e9d779c6db91ef24
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>