Better error handling for static file delivery (#443)

* Improve error messages

* Simplify error handling in deliverPlain()

rqerror.message and rqerror.response.body.toString()
seem to yield the same data, except that rqerror.message
is of type Buffer, so it's contents are printed in the
stream returned via html as a list of bytes (which is
not easy to read)

* increase test coverage

* lint

static.js

@@ -18,10 +18,21 @@ const mime = require('mime-types');
 // one megabyte openwhisk limit + 20% Base64 inflation + safety padding
 const REDIRECT_LIMIT = 750000;
 
+function errorCode(code) {
+  switch (code) {
+    case 400:
+      return 404;
+    default:
+      return code;
+  }
+}
+
 function error(message, code = 500) {
+  // treat
+  const statusCode = errorCode(code);
   console.error('delivering error', message, code);
   return {
-    statusCode: code,
+    statusCode,
     headers: {
       'Content-Type': 'text/html',
       'X-Static': 'Raw/Static',
@@ -129,13 +140,7 @@ function deliverPlain(owner, repo, ref, entry, root) {
         'X-Static': 'Raw/Static',
       },
     };
-  }).catch((rqerror) => {
-    console.error('REQUEST FAILED', rqerror.response.body.toString());
-    if (rqerror.statusCode === 404) {
-      return error(rqerror.response.body.toString(), 404);
-    }
-    return error(rqerror.message, rqerror.statusCode);
-  });
+  }).catch(rqerror => error(rqerror.response.body.toString(), rqerror.statusCode));
 }
 
 /**