Add a pipeline type check to the exec command

[mach6]

If type is not of type 'docker' produce a meaningful
error message. Other pipeline types are not currently
supported by 'drone exec'

The previous error message will likely be
"linter: invalid or missing image", if a
project is using a ssh type, for example.
The error is misleading.

---

example output after this change.

$ ./release/linux/amd64/drone exec                                                                                                                                                                                                                                                                                                                              
2020/07/14 15:15:00 pipeline type (ssh) is not supported with 'drone exec'

[tboerger]

Totally makes sense until we found a way how to properly handle that

[mach6]

@tboerger @bradrydzewski Anything you need from me on the PR? It has been in an approved state for awhile now.