Add error to JsValue conversion and example (#691)

examples/wasm_github_fetch/.gitignore

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+node_modules
+pkg
+target
+Cargo.lock
+*.swp

examples/wasm_github_fetch/Cargo.toml

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+[package]
+name = "wasm"
+version = "0.1.0"
+authors = ["John Gallagher <john.willis.gallagher@gmail.com>"]
+edition = "2018"
+
+# Config mostly pulled from: https://github.com/rustwasm/wasm-bindgen/blob/master/examples/fetch/Cargo.toml
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+reqwest = {path = "../../"}
+serde = { version = "1.0.101", features = ["derive"] }
+serde_derive = "^1.0.59"
+wasm-bindgen-futures = "0.4.1"
+serde_json = "1.0.41"
+wasm-bindgen = { version = "0.2.51", features = ["serde-serialize"]  }

examples/wasm_github_fetch/README.md

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+## Example usage of Reqwest from WASM
+
+You can build the example locally with:
+
+
+    npm run serve
+
+and then visiting http://localhost:8080 in a browser should run the example!
+
+
+This example is loosely based off of [this example](https://github.com/rustwasm/wasm-bindgen/blob/master/examples/fetch/src/lib.rs), an example usage of `fetch` from `wasm-bindgen`.

examples/wasm_github_fetch/index.js

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+const rust = import('./pkg');
+
+rust
+    .then(m => {
+        return m.run().then((data) => {
+            console.log(data);
+
+            console.log("The latest commit to the wasm-bindgen %s branch is:", data.name);
+            console.log("%s, authored by %s <%s>", data.commit.sha, data.commit.commit.author.name, data.commit.commit.author.email);
+        })
+    })
+    .catch(console.error);

examples/wasm_github_fetch/package.json

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+{
+    "scripts": {
+        "build": "webpack",
+        "serve": "webpack-dev-server"
+    },
+    "devDependencies": {
+        "@wasm-tool/wasm-pack-plugin": "1.0.1",
+        "text-encoding": "^0.7.0",
+        "html-webpack-plugin": "^3.2.0",
+        "webpack": "^4.29.4",
+        "webpack-cli": "^3.1.1",
+        "webpack-dev-server": "^3.1.0"
+    }
+}

examples/wasm_github_fetch/src/lib.rs

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+use serde::{Deserialize, Serialize};
+use wasm_bindgen::prelude::*;
+
+// NOTE: This test is a clone of https://github.com/rustwasm/wasm-bindgen/blob/master/examples/fetch/src/lib.rs
+// but uses Reqwest instead of the web_sys fetch api directly
+
+/// A struct to hold some data from the github Branch API.
+///
+/// Note how we don't have to define every member -- serde will ignore extra
+/// data when deserializing
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Branch {
+    pub name: String,
+    pub commit: Commit,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Commit {
+    pub sha: String,
+    pub commit: CommitDetails,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct CommitDetails {
+    pub author: Signature,
+    pub committer: Signature,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Signature {
+    pub name: String,
+    pub email: String,
+}
+
+#[wasm_bindgen]
+pub async fn run() -> Result<JsValue, JsValue> {
+    let res = reqwest::Client::new()
+        .get("https://api.github.com/repos/rustwasm/wasm-bindgen/branches/master")
+        .send()
+        .await?;
+
+    let text = res.text().await?;
+    let branch_info: Branch = serde_json::from_str(&text).unwrap();
+
+    Ok(JsValue::from_serde(&branch_info).unwrap())
+}

examples/wasm_github_fetch/webpack.config.js

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+const path = require('path');
+const HtmlWebpackPlugin = require('html-webpack-plugin');
+const webpack = require('webpack');
+const WasmPackPlugin = require("@wasm-tool/wasm-pack-plugin");
+
+module.exports = {
+    entry: './index.js',
+    output: {
+        path: path.resolve(__dirname, 'dist'),
+        filename: 'index.js',
+    },
+    plugins: [
+        new HtmlWebpackPlugin(),
+        new WasmPackPlugin({
+            crateDirectory: path.resolve(__dirname, ".")
+        }),
+        // Have this example work in Edge which doesn't ship `TextEncoder` or
+        // `TextDecoder` at this time.
+        new webpack.ProvidePlugin({
+            TextDecoder: ['text-encoding', 'TextDecoder'],
+            TextEncoder: ['text-encoding', 'TextEncoder']
+        })
+    ],
+    mode: 'development'
+};