Finding areas that are untested and need love (#4131)

* Makes InstallationRouter like others

* Adds testing for Range file requests

- Fixes issue with small requests (0-2)

* Revert "Makes InstallationRouter like others"

This reverts commit e2d2a16ebf2757db6138c7b5b33c97c56c69ead6.

* Better handling of errors in FilesRouter

* Fix incorrectness in range requests

* Better/simpler logic

* Only on mongo at it requires Gridstore

* Open file streaming to all adapters supporting it

* Improves coverage of parsers

* Ensures depreciation warning is effective

* Removes unused function

* de-duplicate logic

* Removes necessity of overriding req.params.className on subclasses routers

* Use babel-preset-env to ensure min-version compatible code

* removes dead code

* Leverage indexes in order to infer which field is duplicated upon signup

- A note mentioned that it would be possible to leverage using the indexes on username/email to infer which is duplicated

* Small nit

* Better template to match column name

* Restores original implementation for safety

* nits

src/Adapters/Storage/Postgres/PostgresStorageAdapter.js

@@ -945,7 +945,15 @@ export class PostgresStorageAdapter {
       .then(() => ({ ops: [object] }))
       .catch(error => {
         if (error.code === PostgresUniqueIndexViolationError) {
-          throw new Parse.Error(Parse.Error.DUPLICATE_VALUE, 'A duplicate value for a field with unique values was provided');
+          const err = new Parse.Error(Parse.Error.DUPLICATE_VALUE, 'A duplicate value for a field with unique values was provided');
+          err.underlyingError = error;
+          if (error.constraint) {
+            const matches = error.constraint.match(/unique_([a-zA-Z]+)/);
+            if (matches && Array.isArray(matches)) {
+              err.userInfo = { duplicated_field: matches[1] };
+            }
+          }
+          throw err;
         } else {
           throw error;
         }