Simple version check in tonic_build

## Feature Request

### Crates

- tonic
- tonic_build

### Motivation

Something didn't work in my project today, I was getting such errors:

```
error running rustfmt: Os { code: 2, kind: NotFound, message: "No such file or directory" }
```
I spent a lot of time trying to figure out what's going on. This error doesn't tell anything, really.
Eventually, it turned out that this was my mistake, because I had different versions of `tonic` and `tonic_build`:
```
tonic = "0.8"
tonic-build = "0.6"
```

Once I made them the same everything just worked:
```
tonic = "0.8"
tonic-build = "0.8"
```

### Proposal

I know it is [in the docs](https://docs.rs/tonic-build/latest/tonic_build/#required-dependencies) but it's not super easily seen and may be skipped.
Is it possible to perform a simple check if the versions of these 2 libs are the same and if they're not, then throw an appropriate, descriptive error?