godog failed to compile testmain package - but should improve the error message

Please answer these questions before submitting a bug report.

### What version of godog are you using?
```
> godog version
Godog version is: v0.12.0
```

### What version of Go are you using?
```
> go version
go version go1.15.15 linux/amd64
```

### What did you do?

First add a /test folder and executed `godog`.
This provides me godog example steps -> :+1:

Then added the example code in a `test.go` file and ran godog again

### What did you expect to see?
- Execution of the steps and that godog shows the implementation is still `godog.ErrPending`

### What did you see instead?
`failed to compile testmain package: exit status 1 - output: compile: -importcfg: open /tmp/go-build685074617/b001/importcfg.link: no such file or directory`

### Additional context

After lots of struggle, I found out that I had to rename the `test.go` file to `test_test.go` :exclamation:

- now I'm indeed seeing the expected results.

:red_circle: Strong request: Can the error message `failed to compile testmain package: exit status 1` possibly become more helpful, for example telling me that I'm still missing an actual file that ends in `_test.go` ? A more helpful message would have gotten me back on track way sooner then the cryptic error message that is currently being displayed.

Reproduce the failure by creating a test folder with test.go and running godog, then compare it with the working test_test.go case. Start from the godog command path that reports “failed to compile testmain package” and inspect how the compiler output is surfaced. Done means a missing _test.go file produces a more helpful message while the existing working case still runs.

Written by the indexing model from the issue text.