Report error if a dependency's constrain cannot be parsed in the pragma statement

Presently, if a pragma is defined in the script with a constraint that does not match the `v.x.y.z` format a null constrain is returned](https://github.com/grafana/k6deps/blob/main/dependencies.go#L300-L327), instead of reporting as an error.

The following pragme returns a dependency with a null constrain:

```
"use k6 > v1.0.0-rc"
```
 