For `node_is_alive`, it seemed to be a good idea to exit with a
`CRITICAL` when the target doesn't exist. But for all the rest, UNKNOWN
(which corresponds to a configuration error) seems better.
Previously if a node wasn't reachable whe would get an UNKNOWN error.
instead of a CRITICAL error.
```
NODEISALIVE UNKNOWN - Connection failed for all provided endpoints
```
We now get the correct error.
```
NODEISALIVE CRITICAL - This node is not alive (patroni is not running). | is_alive=0;;@0
```
Stop using ctx.parent.params to get the verbose and timeout parameters
parsed in main and use ctx.obj instead.
ctx.parent.params is typed as Optional[Context] which forces us to test
if it's NULL before using it. It's useless in our case because we know
it's not empty and the resulting code is ugly.
The mypy ierror.
Item "None" of "Optional[Context]" has an attribute "params"
[union-attr]
