suggest wrangler init if wrangler.toml does not exist

src/settings/toml/manifest.rs

@@ -45,6 +45,10 @@ pub struct Manifest {
 
 impl Manifest {
     pub fn new(config_path: &Path) -> Result<Self, failure::Error> {
+        failure::ensure!(
+            config_path.exists(),
+            "wrangler.toml not found; run `wrangler init` to create one."
+        );
         let config = read_config(config_path)?;
 
         let manifest: Manifest = match config.try_into() {

src/settings/toml/tests/mod.rs

@@ -146,5 +146,5 @@ fn base_fixture_path() -> PathBuf {
 }
 
 fn toml_fixture_path(fixture: &str) -> PathBuf {
-    base_fixture_path().join(fixture)
+    base_fixture_path().join(fixture).with_extension("toml")
 }