postcss: Fix import error handling

Note that we will now fail if `inlineImports` is enabled and we cannot resolve an import.

You can work around this by either:

* Use url imports or imports with media queries.
* Set `skipInlineImportsNotFound=true` in the options

Also get the argument order in the different NewFileError* funcs in line.

Fixes #9895

hugofs/fileinfo.go

@@ -139,6 +139,17 @@ type fileInfoMeta struct {
 	m *FileMeta
 }
 
+type filenameProvider interface {
+	Filename() string
+}
+
+var _ filenameProvider = (*fileInfoMeta)(nil)
+
+// Filename returns the full filename.
+func (fi *fileInfoMeta) Filename() string {
+	return fi.m.Filename
+}
+
 // Name returns the file's name. Note that we follow symlinks,
 // if supported by the file system, and the Name given here will be the
 // name of the symlink, which is what Hugo needs in all situations.