Add more meaningful error when rescript.json is not found.

Author: nojaf

rewatch/src/build/clean.rs

@@ -67,7 +67,7 @@ fn clean_source_files(build_state: &BuildState, root_package: &packages::Package
             SourceType::SourceFile(source_file) => {
                 let package = build_state.packages.get(&module.package_name).unwrap_or_else(|| {
                     panic!(
-                        "Could not find package for \"{}\" in build state",
+                        "Could not find package for \"{}\" in build state.\nMaybe you forgot to add the relative path to your root rescript.config.json file?",
                         &module.package_name
                     )
                 });

rewatch/src/build/packages.rs

@@ -244,8 +244,13 @@ pub fn read_config(package_dir: &Path) -> Result<config::Config> {
 
     if Path::new(&rescript_json_path).exists() {
         config::Config::new(&rescript_json_path)
-    } else {
+    } else if Path::new(&bsconfig_json_path).exists() {
         config::Config::new(&bsconfig_json_path)
+    } else {
+        Err(anyhow!(
+            "Could not find rescript.json or bsconfig.json in package directory: {}",
+            package_dir.to_string_lossy()
+        ))
     }
 }