Use HTTP code of underlying errors

src/__tests__/http-test.ts

@@ -1462,7 +1462,7 @@ function runTests(server: Server) {
         .set('Content-Type', 'application/json')
         .send(`{ "query": "{ ${new Array(102400).fill('test').join('')} }" }`);
 
-      expect(response.status).to.equal(400);
+      expect(response.status).to.equal(413);
       expect(JSON.parse(response.text)).to.deep.equal({
         errors: [{ message: 'Invalid body: request entity too large.' }],
       });

src/parseBody.ts

@@ -100,10 +100,18 @@ async function readBody(
   // Read body from stream.
   try {
     return await getBody(stream, { encoding: charset, length, limit });
-  } catch (err) {
-    throw err.type === 'encoding.unsupported'
-      ? httpError(415, `Unsupported charset "${charset.toUpperCase()}".`)
-      : httpError(400, `Invalid body: ${String(err.message)}.`);
+  } catch (rawError: unknown) {
+    const error = httpError(
+      400,
+      /* istanbul ignore next: Thrown by underlying library. */
+      rawError instanceof Error ? rawError : String(rawError),
+    );
+
+    error.message =
+      error.type === 'encoding.unsupported'
+        ? `Unsupported charset "${charset.toUpperCase()}".`
+        : `Invalid body: ${error.message}.`;
+    throw error;
   }
 }