Improve error messaging in YangPatch

Improve error messaging in YangPatch

The method readValueNodes returns exception message
"Error parsing input" for several error cases.
Fix this by handling each error case individually with a proper
error message.

Fix ErrorType from PROTOCOL to APLICATION, since this error
is not related to the NETCONF protocol.

Fix ErrorTag from MALFORMED_MESSAGE to MISSING_ELEMENT when 'value'
element is required and missing, and to UNKNOWN_ELEMENT when 'value'
element is provided for DELETE, MOVE and REMOVE operation.

Fix JsonPatchBodyTest target path to throw correct exception.

JIRA: NETCONF-1438
Change-Id: I9226ede2d664c1c8c24db1bccc7f412ab929eb34
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>