Make SvgParseError non_exhaustive

linebender#329 was a breaking change
To avoid a similar situation in future, mark the offending enum
`#[non_exhaustive]`

src/svg.rs

@@ -239,6 +239,7 @@ impl BezPath {
 
 /// An error which can be returned when parsing an SVG.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum SvgParseError {
     /// A number was expected.
     Wrong,