Log fx's startup steps at debug; a refused setting is one line (closes #209)

fx's event lines, errors included, now log at debug level, so a normal
start shows none and DEBUG shows them. RunServer starts and stops the
fx app itself instead of calling Run, and logs fx's "received signal"
line as Run did. When the start fails, it logs the error's root cause
as one "cannot start" line, the backend's last, and returns 1. Main
returns the server's exit code.

Model: opus-5-5

internal/cli/cli_test.go

@@ -63,7 +63,7 @@ func TestMainRunsPrepareDataDir(t *testing.T) {
 
 // TestMainRunsServerWithNoArguments: Main given no arguments starts the
 // server with the name and version it was given, writes nothing and
-// returns 0.
+// returns the server's exit code.
 func TestMainRunsServerWithNoArguments(t *testing.T) {
 	t.Parallel()
 
@@ -73,11 +73,13 @@ func TestMainRunsServerWithNoArguments(t *testing.T) {
 	)
 
 	code := cli.Main(&stderr, "netwatch", "test", []string{},
-		func(appname, version string) {
+		func(appname, version string) int {
 			gotAppname, gotVersion = appname, version
+
+			return 3
 		})
-	if code != 0 || stderr.Len() != 0 {
-		t.Errorf("exit %d, stderr %q; want exit 0 and no output",
+	if code != 3 || stderr.Len() != 0 {
+		t.Errorf("exit %d, stderr %q; want exit 3 and no output",
 			code, stderr.String())
 	}
 
@@ -210,10 +212,12 @@ func currentUsername(t *testing.T) string {
 
 // failIfServerStarts is what the tests give Main in place of
 // cli.RunServer: it fails the test instead of starting the server.
-func failIfServerStarts(t *testing.T) func(appname, version string) {
+func failIfServerStarts(t *testing.T) func(appname, version string) int {
 	t.Helper()
 
-	return func(string, string) {
+	return func(string, string) int {
 		t.Error("Main started the server")
+
+		return 0
 	}
 }