Rename grpcRun to grpcRunE: the inner closure now returns error
and the wrapper returns a cobra RunE-compatible function.
Change newHeadscaleCLIWithConfig to return an error instead of
calling log.Fatal/os.Exit, making connection failures propagate
through the normal error path.
Add formatOutput (returns error) and printOutput (writes to stdout)
as non-exiting replacements for the old output/SuccessOutput pair.
Extract output format string literals into package-level constants.
Mark the old ErrorOutput, SuccessOutput and output helpers as
deprecated; they remain temporarily for the unconverted commands.
Convert all 22 grpcRunE commands from Run+ErrorOutput+SuccessOutput
to RunE+fmt.Errorf+printOutput. Change usernameAndIDFromFlag to
return an error instead of calling ErrorOutput directly.
Update backfillNodeIPsCmd and policy.go callers of
newHeadscaleCLIWithConfig for the new 5-return signature while
keeping their Run-based pattern for now.
e4fe216e45