Improvements to tests and FromHexError

[Luro02]

• use pretty_assertions (highlights the differences, when something mismatches)
• increased test coverage
• removed soft-deprecated description function of FromHexError
• fixed minor problem with the fmt, where the char would be printed out, this doesn't make sense because you would get error messages like this one:

  Invalid character 
  at position 5

  (\n at position 5)

[KokaKiwi]

Sorry for late review, was busy lately with stuff x_x Thanks for your contribution ^-^