fix(web): return startup errors instead of panicking (#1620)

Replace server startup panics with returned errors, print actionable CLI
messages with exit status 1, and clarify the API token flags.

Add regressions for missing web authentication credentials.

Fixes GopeedLab/gopeed#1614

cmd/api/main.go

@@ -1,6 +1,9 @@
 package main
 
 import (
+	"fmt"
+	"os"
+
 	"github.com/GopeedLab/gopeed/cmd"
 	"github.com/GopeedLab/gopeed/pkg/rest/model"
 )
@@ -13,5 +16,8 @@ func main() {
 		Storage:   model.StorageBolt,
 		WebEnable: true,
 	}
-	cmd.Start(cfg)
+	if err := cmd.Start(cfg); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
 }