Pull request: all: imp code, err handling

Closes #2571.

Squashed commit of the following:

commit a5b50ee011a995f4ab3d93314acd6f0ca82d99cf
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Mar 15 14:05:25 2021 +0300

    all: imp code

commit bc610f8f438549e8c6b04c8a213b5422dda2aff5
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Mar 12 20:00:14 2021 +0300

    all: imp code, err handling

internal/home/mobileconfig.go

@@ -58,7 +58,7 @@ func getMobileConfig(d dnsSettings) ([]byte, error) {
 		dspName = fmt.Sprintf("%s DoH", d.ServerName)
 
 		u := &url.URL{
-			Scheme: "https",
+			Scheme: schemeHTTPS,
 			Host:   d.ServerName,
 			Path:   "/dns-query",
 		}