`cabal clean` should fail if not in a cabal directory

**Describe the bug**
Currently the `cabal clean` command will silently fail if the directory is not a cabal one. This can be confusing in some situations. I've asked people to "`cabal clean` and try again", and turns out they were performing this in the root directory of the whole project, rather than the directory containing our Haskell code. If `cabal clean` produced an error in such situations, this would have helped.

**To Reproduce**
Call `cabal clean` in a non-cabal directory.

**Expected behavior**
The same behaviour as e.g. `cabal build`:
```
Error: cabal: There is no .cabal package file or cabal.project file.
```