Add more context to a wasm-bindgen-test-runner error

Minor cleanup I found during #1002

crates/cli/src/bin/wasm-bindgen-test-runner/main.rs

@@ -155,7 +155,7 @@ fn rmain() -> Result<(), Error> {
         &tmpdir,
         &args.collect::<Vec<_>>(),
         &tests,
-    )?;
+    ).context("failed to spawn server")?;
     let addr = srv.server_addr();
 
     // TODO: eventually we should provide the ability to exit at some point