feat: improved indentation in error messages

[baszalmstra]

I extracted the new indentation code from #28

Old

The following packages are incompatible
|-- a * cannot be installed because there are no viable options:
    |-- a 2 would require
        |-- b *, which cannot be installed because there are no viable options:
            |-- b 1 is excluded because it is externally excluded
    |-- a 1 would require
        |-- c *, which cannot be installed because there are no viable options:
            |-- c 1 is excluded because it is externally excluded

New

The following packages are incompatible
└─ a * cannot be installed because there are no viable options:
   ├─ a 2 would require
   │  └─ b *, which cannot be installed because there are no viable options:
   │     └─ b 1 is excluded because it is externally excluded
   └─ a 1 would require
      └─ c *, which cannot be installed because there are no viable options:
         └─ c 1 is excluded because it is externally excluded

[wolfv]

Looking gooood!