Annotation edgedb version that generates migration

At the start of migration add:
```
# Generated by EdgeDB version: v3.0-alpha.2
```

Then, when reading migrations do two things:
1. Warn if current edgedb version is lower than that
2. If migration cannot be applied, add the check to context of the error kinda:
```
ScheamDefinitionError: something gone wrong
[..100 lines of error descripiton..]
Note: the migration created by EdgeDB v3.0-alpha.2, current EdgeDB is v2.12
```