Poor error for duplicate files

[sagehane]

Issue:

Running commands when multiple inputs generate the same file will result in the following error message:

Unhandled error: error.DuplicateFile

This is a bug in Zangle, please report it on the issue tracker
along with any information that may help in resolving the bug.

  https://github.com/tauoverpi/zangle/issues/new

Expected behaviour:

The error-case should actually be handled and it should state which files were duplicated.

How to reproduce:

1. Provide multiple files that generate identical files as args
2. Run commands like zangle ls <files>

Using a directory with two different files generating the same file:

$ zangle ls foobar
Unhandled error: error.DuplicateFile

This is a bug in Zangle, please report it on the issue tracker
along with any information that may help in resolving the bug.

  https://github.com/tauoverpi/zangle/issues/new

Using the same file twice:

$ zangle ls foobar/foo.md foobar/foo.md
Unhandled error: error.DuplicateFile

This is a bug in Zangle, please report it on the issue tracker
along with any information that may help in resolving the bug.

  https://github.com/tauoverpi/zangle/issues/new

Sample fies:

foobar/foo.md:

```txt file: foobar.md
foo
```

foobar/bar.md:

```txt file: foobar.md
bar
```