Improve panic messages if retrieving/parsing GL_VERSION fails (#331)

* Improve panic messages if retrieving/parsing GL_VERSION fails

* Format

src/web_sys.rs

@@ -253,8 +253,14 @@ impl Context {
         let (extensions, supported_extensions) = build_extensions!(context, WebGlRenderingContext);
 
         // Retrieve and parse `GL_VERSION`
-        let raw_string = context.get_parameter(VERSION).unwrap().as_string().unwrap();
-        let version = Version::parse(&raw_string).unwrap();
+        let raw_jsvalue = context
+            .get_parameter(VERSION)
+            .expect("context.get_parameter(VERSION) shouldn't throw");
+        let raw_string = raw_jsvalue
+            .as_string()
+            .unwrap_or_else(|| panic!("{:?} should be a string", raw_jsvalue));
+        let version = Version::parse(&raw_string)
+            .expect("context.get_parameter(VERSION) should be parseable as an OpenGL version");
 
         Self {
             raw: RawRenderingContext::WebGl1(context),
@@ -279,8 +285,14 @@ impl Context {
         let (extensions, supported_extensions) = build_extensions!(context, WebGl2RenderingContext);
 
         // Retrieve and parse `GL_VERSION`
-        let raw_string = context.get_parameter(VERSION).unwrap().as_string().unwrap();
-        let version = Version::parse(&raw_string).unwrap();
+        let raw_jsvalue = context
+            .get_parameter(VERSION)
+            .expect("context.get_parameter(VERSION) shouldn't throw");
+        let raw_string = raw_jsvalue
+            .as_string()
+            .unwrap_or_else(|| panic!("{:?} should be a string", raw_jsvalue));
+        let version = Version::parse(&raw_string)
+            .expect("context.get_parameter(VERSION) should be parseable as an OpenGL version");
 
         Self {
             raw: RawRenderingContext::WebGl2(context),