Replace EBML parsing error code (u8) with error enum

[FreezyLemon]

This is a suggestion to make the EBML errors better. Feel free to change things or just let me know what you think.

Changes:

1. Introduce ebml::EbmlError enum to represent errors instead of u8 error codes.
2. Change some of the error code values (the values 1 and 104 were used ambiguously)
3. Make the helper function custom_error return a nom:Err (specifically, the recoverable Error variant) wrapped around the ebml::Error instead of just the ebml::Error. This simplifies the function calls.

I thought about #[repr(u8)] for the new enum, but realistically the enclosing ebml::Error struct will be padded anyways, so it probably doesn't matter if the enum is represented as a u16 or even larger in the future.

I'm not sure about the actual error codes. If the numerical values are supposed to be expressive, it would make sense to think of some kind of numbering scheme for the error codes before merging. Otherwise, the numerical values can (IMO) just be removed.

[lu-zero]

Thank you!