(@graphql-tools/load-files) - improve error message on import to require fallback

**Problem description.**
Hello!
I'm rewriting my app from common js to es6 modules. So I have to move from loadFileSync to async loadFile to import my schemas and resolvers. The problem I've encountered is that if you have any troubles with your imports within resolvers, you get the error, on this [line](https://github.com/ardatan/graphql-tools/blob/ed367305beba668fb44b0a3baa13a946a522eb3b/packages/load-files/src/index.ts#L228):
```javascript
const defaultRequireMethod = (path) => import(path).catch(async () => require(path));
^
ReferenceError: require is not defined
at file:///home/jackson/work/projects/fasty/node_modules/@graphql-tools/load-files/index.mjs:155:75
at async file:///home/jackson/work/projects/fasty/node_modules/@graphql-tools/load-files/index.mjs:162:33
at async Promise.all (index 0)
at async default (file:///home/jackson/work/projects/fasty/server/src/server/graphql/index.js:17:36)
at async default (file:///home/jackson/work/projects/fasty/server/src/server/index.js:7:35)
at async file:///home/jackson/work/projects/fasty/server/bin/index.js:10:15

```
Import fails due to errors in code and fallback to require. Obviously require is not defined cause I'm using es6 modules. I've spent some time figuring out what's wrong, cause error really doesn't help much to locate a problem.

**The solution that helped me:**
Simply added console.log of error thrown by import fail and got all the problems causing that:
```javascript
const defaultRequireMethod = (path: string) => import(path).catch(async (err) => {
console.log(err)
return require(path)
});
```

**Request:**
Make import fail errors more clear and informable.

**Question:**
Why fallback to ```require``` anyway, as using import with es6 modules will always lead to ```undefined require``` error? I'm a beginner programmer so the question is really to learn and understand. Thanks for your reply in advance.

Start at packages/load-files/src/index.ts around line 228, where the async import fallback is defined. Reproduce an import failure and inspect how the caught error is replaced by the fallback error; done means the original import failure is exposed clearly instead of only reporting that require is unavailable.

Written by the indexing model from the issue text.