Print the format type when the format is in an incorrent position

[boozec]

When it raises an error caused by position of the format, now prints the format type as string instead of the position inside the array of formats.
In this way you can read on stdout the type like .tar or .lz instead of 1, 2, .., n.

Example

Current code:

$ ./target/debug/ouch c hello.txt archive.tar.lz.tar.lz
[ERROR] Cannot compress to 'archive.tar.lz.tar.lz'.
 - Found the format '1' in an incorrect position.
 - 1 can only be used at the start of the file extension.

hint: If you wish to compress multiple files, start the extension with 1.
hint: Otherwise, remove 1 from 'archive.tar.lz.tar.lz'.

What fixs this PR:

$ ./target/debug/ouch c hello.txt archive.tar.lz.tar.lz
[ERROR] Cannot compress to 'archive.tar.lz.tar.lz'.
 - Found the format '.lz' in an incorrect position.
 - '.lz' can only be used at the start of the file extension.

hint: If you wish to compress multiple files, start the extension with '.lz'.
hint: Otherwise, remove '.lz' from 'archive.tar.lz.tar.lz'.

[marcospb19]

Thanks for opening a PR!

[marcospb19]

The new message:

[ERROR] Cannot compress to 'archive.tar.lz.tar.lz'.
- Found the format '.lz' in an incorrect position.
- '.lz' can only be used at the start of the file extension.

The error message expected would be:

> [ERROR] Cannot compress to 'archive.tar.lz.tar.lz'.
> - Found the format '.tar' in an incorrect position.
> - '.tar' can only be used at the start of the file extension.

And this would be referring only to the second .tar part, because it makes no sense creating a tar archive with only one file, we made it into an error (but maybe it shouldn't be? What do you think??).

[marcospb19]

Resolved conflicts, ready to merge (waiting for CI).

[marcospb19]

Moved pending discussion about the conflicting error messages to #88.