feat: Raise error if waved cannot find public/private dir #2130 (#2378)

Author: dbranley

server.go

@@ -22,6 +22,7 @@ import (
 	"io/ioutil"
 	"log"
 	"net/http"
+	"os"
 	"path"
 	"path/filepath"
 	"strings"
@@ -169,6 +170,10 @@ func splitDirMapping(m string) (string, string) {
 		panic(fmt.Sprintf("invalid directory mapping: want \"remote@local\", got %s", m))
 	}
 
+	if _, err := os.Stat(xs[1]); os.IsNotExist(err) {
+		panic(fmt.Sprintf("directory does not exist: %s", xs[1]))
+	}
+
 	// Windows prepends the drive letter to the path with a leading slash, e.g. "/foo/" => "C:/foo/".
 	if xs[0][1] == ':' {
 		xs[0] = xs[0][2:]