Design consistent user friendly error output

[danielpclark]

Errors have inconsistent behavior. Create a nice experience.

Structure the main application so all errors will produce a relevant error description to an end user. No errors are to be displayed relating to code in a released binary. Errors should be pleasant to read… as in easy on the eyes and easy to understand.

[danielpclark]

Current Errors

1) Invalid Range

Input:

abrute asdf asdf -- asdf

Output:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ParseIntError { kind: InvalidDigit }', /checkout/src/libcore/result.rs:906:4

2) File Missing

Input:

abrute 4 asdf -- asdf

Output:

Error: File "asdf" does not exist.
error: "Please verify last argument is the proper filename."

3) Missing Double Dash

Input:

abrute 4 asdf asdf

Output:

error: Found argument 'asdf' which wasn't expected, or isn't valid in this context

USAGE:
    abrute [OPTIONS] <RANGE> <CHARACTERS> [--] <TARGET>

For more information try --help

Note the options are in the wrong location and the dash dash should not be boxed in this message.

4) Invalid Adjacent Number

Input:

abrute 4 asdf -a g -- asdf

Output:

error: Invalid value for '--adjacent <adjacent>': The value did not contain valid digit from 0 to 255

5) Invalid Character Start Sequence

Using characters that don't exist in the sequence for the start.

Input:

abrute 4 asdf -s 1234 -- asdf

Output:

thread 'main' panicked at 'no entry found for key', /checkout/src/libcore/option.rs:839:4