fix: improve error reporting when port is in use

Before this change, when we could not start listening on webserver's TCP
port, the binary produced by GoReleaser would abort with no meaningful
error report:

    ```
    ❯ ./build/saturn/l2node-darwin-arm64/saturn-l2
    [1]    16654 killed     ./build/saturn/l2node-darwin-arm64/saturn-l2
    ```

In this commit, I am reworking error handling to print the error to
stderr and exit the process with a non-zero exit code via `os.Exit`.

Signed-off-by: Miroslav Bajtoš <saturn@bajtos.net>

main/main.go

@@ -58,7 +58,8 @@ func main() {
 
 	nl, err := net.Listen("tcp", fmt.Sprintf(":%d", port))
 	if err != nil {
-		panic(err)
+		fmt.Fprintf(os.Stderr, "Cannot start the webserver: %s\n", err.Error())
+		os.Exit(4)
 	}
 
 	go func() {