bluetooth: host: hci_core: Safer checking of conn complete events

The spec only guarantees the connection complete event parameters
are valid if the status is BT_HCI_ERR_SUCCESS. When the status is
not BT_HCI_ERR_SUCCESS, the host shall ignore the other parameters.

See Vol 4, Part E, 4.5 Command error handling:
"""
If an error occurs for a command for which an HCI_Command_Complete event
is returned, the Return Parameters field may only contain some of the
return parameters specified for the command.
...
The above also applies to commands that have associated command specific
completion events with a Status parameter in their completion event, with
the exceptions shown in Table 4.1, Event Valid parameters
...
Event                           | Valid parameters
------------------------------------------------------------
LE_Connection_Complete          | none
LE_Enhanced_Connection_Complete | none
"""

Refactor `le_legacy_conn_complete`, `le_enh_conn_complete_v2` and
`le_enh_conn_complete` to check and handle the status before handling
any other parameters.

An issue was seen where SDC returned event with status
`BT_HCI_ERR_UNKNOWN_CONN_ID`, but because adv_handle and sync_handle
were not invalid the event was not handled.

Signed-off-by: Sean Madigan <sean.madigan@nordicsemi.no>