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/basicfs.go

@@ -19,8 +19,11 @@ import (
 )
 
 var (
-	ErrInvalidFilename = errors.New("filename is invalid")
-	ErrNotRelative     = errors.New("not a relative path")
+	errInvalidFilenameEmpty               = errors.New("name is invalid, must not be empty")
+	errInvalidFilenameWindowsSpacePeriod  = errors.New("name is invalid, must not end in space or period on Windows")
+	errInvalidFilenameWindowsReservedName = errors.New("name is invalid, contains Windows reserved name (NUL, COM1, etc.)")
+	errInvalidFilenameWindowsReservedChar = errors.New("name is invalid, contains Windows reserved character (?, *, etc.)")
+	errNotRelative                        = errors.New("not a relative path")
 )
 
 func WithJunctionsAsDirs() Option {
@@ -95,7 +98,7 @@ func (f *BasicFilesystem) rooted(rel string) (string, error) {
 func rooted(rel, root string) (string, error) {
 	// The root must not be empty.
 	if root == "" {
-		return "", ErrInvalidFilename
+		return "", errInvalidFilenameEmpty
 	}
 
 	var err error