Improve error messages

[yannham]

Partly address #92. Depends on #97. This PR adds more specific variants to the error type EvalError, and moves the elaboration of error messages to a dedicated location, namely the implementation of the trait defined in #97. The evaluation code now passes elementary data through these variants rather than building error messages itself, wherever possible. The intended result is that error messages are more consistent and easier to improve in the future.

Examples

Doing so, error messages have also been revamped although was not the main focus, and can still be improved. Blame error messages have not yet been modified. Here are a few examples of messages before and after this PR:

Before

% nickel <<< '"a" 1'
Evaluation didn't finished, found error:
The term Str("a") was applied to RichTerm { term: Num(1.0), pos: Some((882, 883)) }

% nickel <<< 'merge 2 {a=1;}'
Evaluation didn't finished, found error:
Could not merge Num(2.0) and Record({Ident("a"): RichTerm { term: Num(1.0), pos: Some((889, 890)) }})

% nickel <<< 'blame 1'
Evaluation didn't finished, found error:
Expected Label, got Num(1.0)

% nickel <<< '{a=1;}.b'
Evaluation didn't finished, found error:
Record didn't have field Ident("b")

Now

% nickel <<< '"a" 1'
Error: At <stdin> line 1, columns 1-6: tried to apply "a" (from <stdin> line 1, columns 1-4) to 1 (from <stdin> line 1, columns 5-6),
but this is not a function.`

% nickel <<< 'merge 2 {a=1;}'
Error: At <stdin> line 1, columns 1-15: could not merge 2 (from <stdin> line 1, columns 7-8) with { ... } (from <stdin> line 1, column
s 9-15)

% nickel <<< 'blame 1'
Error: At <stdin> line 1, columns 7-8: expected Label, got Num (blame)

% nickel <<< '{a=1;}.b'
Error: At <stdin> line 1, columns 1-9: field "b" is required by (.), but was not found in the record at <stdin> line 1, columns 1-7

[yannham]

Superseded by #102.