feat: improve JSON errors debuggability

As seen in #393 previously
JSON decoding errors are obscure.

This changes improve the debugging in two cases:
  - The `package.json` file fails to decode; now emits a clearer error.
  - The wranglerjs backend returns an invalid JSON; now preserves the
  output file for further investigation.
    The console doesn't print the output file location by default, you
    need to pass `RUST_LOG=info` while running `wrangler build` and
    search for `--output-file=FILE` argument passed to wranglerjs.

src/commands/build/wranglerjs/mod.rs

@@ -35,7 +35,6 @@ pub fn run_build(project: &Project) -> Result<(), failure::Error> {
 
     if status.success() {
         let output = fs::read_to_string(temp_file.clone()).expect("could not retrieve ouput");
-        fs::remove_file(temp_file)?;
 
         let wranglerjs_output: WranglerjsOutput =
             serde_json::from_str(&output).expect("could not parse wranglerjs output");
@@ -54,10 +53,10 @@ pub fn run_build(project: &Project) -> Result<(), failure::Error> {
             wranglerjs_output.project_size()
         );
 
+        fs::remove_file(temp_file)?;
         message::success(&msg);
         Ok(())
     } else {
-        fs::remove_file(temp_file)?;
         failure::bail!("failed to execute `{:?}`: exited with {}", command, status)
     }
 }

src/commands/publish/package.rs

@@ -36,8 +36,9 @@ impl Package {
             )
         }
 
-        let package_json: String = fs::read_to_string(manifest_path)?.parse()?;
-        let package: Package = serde_json::from_str(&package_json)?;
+        let package_json: String = fs::read_to_string(manifest_path.clone())?.parse()?;
+        let package: Package = serde_json::from_str(&package_json)
+            .expect(&format!("could not parse {:?}", manifest_path));
 
         Ok(package)
     }