fix: exit non-zero when CLI flag/env parsing fails

[stareezy-1]

Summary

cloudflared exits with status 0 and no output when a flag or environment
variable fails to parse, e.g. TUNNEL_GRACE_PERIOD=10 (see #1606).

Problem

TUNNEL_GRACE_PERIOD=10 (missing the duration unit) fails
time.ParseDuration inside urfave/cli's flag setup. App.Run returns that
error without printing anything, and all four runApp implementations
discarded the returned error:

app.Run(os.Args)      // generic, windows (interactive)
_ = app.Run(os.Args)  // linux, macos

so the process exited 0 silently. The same discard made the CLI case
(cloudflared tunnel --grace-period 10 run) exit 0 after printing
"Incorrect Usage", which breaks scripts and service managers that rely on
the exit status. The issue also reports systemd reporting a confusing
"protocol" failure.

Fix

• New runAppAndExit helper (cmd/cloudflared/run_app.go): runs the app,
  prints any returned error to stderr, and exits 1.
• All four runApp implementations (generic/linux/macos/windows) now use
  it instead of discarding the error.

Behavior before → after:

Invocation	Before	After
TUNNEL_GRACE_PERIOD=10 cloudflared tunnel run	exit 0, no output	exit 1, could not parse "10" as duration value for flag grace-period: time: missing unit in duration "10"
cloudflared tunnel --grace-period 10 run	exit 0	exit 1
cloudflared --version / --help	exit 0	exit 0 (unchanged)

Tests

• New regression test TestRunAppInvalidGracePeriodEnvExitsNonZero
  (cmd/cloudflared/main_test.go): re-executes the test binary with
  TUNNEL_GRACE_PERIOD=10 via the helper-process pattern and asserts a
  non-zero exit code plus the error message. Fails without the fix (exit 0),
  passes with it.
• make test (go test -race ./...): 940 packages pass, 0 failures.
• go vet ./cmd/cloudflared/ clean; golangci-lint reports no new issues
  in the touched files.