Moved back pyscript.core where it belongs (#1622)

pyscript.core/src/fetch.js

@@ -0,0 +1,63 @@
+import { FetchError, ErrorCode } from "./exceptions.js";
+
+/**
+ * This is a fetch wrapper that handles any non 200 responses and throws a
+ * FetchError with the right ErrorCode. This is useful because our FetchError
+ * will automatically create an alert banner.
+ *
+ * @param {string} url - URL to fetch
+ * @param {Request} [options] - options to pass to fetch
+ * @returns {Promise<Response>}
+ */
+export async function robustFetch(url, options) {
+    let response;
+
+    // Note: We need to wrap fetch into a try/catch block because fetch
+    // throws a TypeError if the URL is invalid such as http://blah.blah
+    try {
+        response = await fetch(url, options);
+    } catch (err) {
+        const error = err;
+        let errMsg;
+        if (url.startsWith("http")) {
+            errMsg =
+                `Fetching from URL ${url} failed with error ` +
+                `'${error.message}'. Are your filename and path correct?`;
+        } else {
+            errMsg = `Polyscript: Access to local files
+        (using [[fetch]] configurations in &lt;py-config&gt;)
+        is not available when directly opening a HTML file;
+        you must use a webserver to serve the additional files.
+        See <a style="text-decoration: underline;" href="https://github.com/pyscript/pyscript/issues/257#issuecomment-1119595062">this reference</a>
+        on starting a simple webserver with Python.
+            `;
+        }
+        throw new FetchError(ErrorCode.FETCH_ERROR, errMsg);
+    }
+
+    // Note that response.ok is true for 200-299 responses
+    if (!response.ok) {
+        const errorMsg = `Fetching from URL ${url} failed with error ${response.status} (${response.statusText}). Are your filename and path correct?`;
+        switch (response.status) {
+            case 404:
+                throw new FetchError(ErrorCode.FETCH_NOT_FOUND_ERROR, errorMsg);
+            case 401:
+                throw new FetchError(
+                    ErrorCode.FETCH_UNAUTHORIZED_ERROR,
+                    errorMsg,
+                );
+            case 403:
+                throw new FetchError(ErrorCode.FETCH_FORBIDDEN_ERROR, errorMsg);
+            case 500:
+                throw new FetchError(ErrorCode.FETCH_SERVER_ERROR, errorMsg);
+            case 503:
+                throw new FetchError(
+                    ErrorCode.FETCH_UNAVAILABLE_ERROR,
+                    errorMsg,
+                );
+            default:
+                throw new FetchError(ErrorCode.FETCH_ERROR, errorMsg);
+        }
+    }
+    return response;
+}