Clarify error message when host drive path is inaccessible

Example with firectl:

$ ./firectl --kernel=./vmlinux
WARN[0000] Failed handler "validate.Cfg": failed to stat host path, "": stat : no such file or directory
FATA[0000] Failed to start machine: failed to stat host path, "": stat : no such file or directory

As you can see it's rather impossible for the user to tell what exactly is missing.

Signed-off-by: Nikolay Edigaryev <[email protected]>

Author: edigaryev

machine.go

@@ -177,7 +177,7 @@ func (cfg *Config) Validate() error {
 		if BoolValue(drive.IsRootDevice) {
 			rootPath := StringValue(drive.PathOnHost)
 			if _, err := os.Stat(rootPath); err != nil {
-				return fmt.Errorf("failed to stat host path, %q: %v", rootPath, err)
+				return fmt.Errorf("failed to stat host drive path, %q: %v", rootPath, err)
 			}
 
 			break