lib/fs: Be more clear about invalid file names (ref #7010) (#7011)

Add specific errors for the failures, resulting in this rather than just
the generic "invalid filename":

[MRIW7] 08:50:50 INFO: Puller (folder default, item "NUL"): syncing: filename is invalid: name is reserved
[MRIW7] 08:50:50 INFO: Puller (folder default, item "fail."): syncing: filename is invalid: name ends with space or period
[MRIW7] 08:50:50 INFO: Puller (folder default, item "sup:yo"): syncing: filename is invalid: name contains reserved character
[MRIW7] 08:50:50 INFO: default: Failed to sync 3 items

lib/fs/filesystem.go

@@ -234,7 +234,7 @@ func Canonicalize(file string) (string, error) {
 		// The relative path may pretend to be an absolute path within
 		// the root, but the double path separator on Windows implies
 		// something else and is out of spec.
-		return "", ErrNotRelative
+		return "", errNotRelative
 	}
 
 	// The relative path should be clean from internal dotdots and similar
@@ -244,10 +244,10 @@ func Canonicalize(file string) (string, error) {
 	// It is not acceptable to attempt to traverse upwards.
 	switch file {
 	case "..":
-		return "", ErrNotRelative
+		return "", errNotRelative
 	}
 	if strings.HasPrefix(file, ".."+pathSep) {
-		return "", ErrNotRelative
+		return "", errNotRelative
 	}
 
 	if strings.HasPrefix(file, pathSep) {