Replace unwrap in error handling (#93)

I ran accross a case where instead of a warning I got an unwrap in error
handling. Two cases didn't seem to use `unwrap_or(0..0)` like all the
others in this function. The errors will get slightly weird when it's
missing but at least it won't cause crashes and the user has some idea
of what is wrong

src/compose/error.rs

@@ -195,7 +195,7 @@ impl ComposerError {
                     .map(|(span, desc)| {
                         trace!(
                             "mapping span {:?} -> {:?}",
-                            span.to_range().unwrap(),
+                            span.to_range().unwrap_or(0..0),
                             map_span(span.to_range().unwrap_or(0..0))
                         );
                         Label::primary((), map_span(span.to_range().unwrap_or(0..0)))
@@ -217,7 +217,8 @@ impl ComposerError {
             ComposerErrorInner::WgslParseError(e) => (
                 e.labels()
                     .map(|(range, msg)| {
-                        Label::primary((), map_span(range.to_range().unwrap())).with_message(msg)
+                        Label::primary((), map_span(range.to_range().unwrap_or(0..0)))
+                            .with_message(msg)
                     })
                     .collect(),
                 vec![e.message().to_owned()],