Simplify and improve the error argument of Rephraser

[janluke]

The error argument is of type Union[None, str, ErrorRephraser].

ErrorRephraser was redefined (breaking change) from:

(ctx: Context, constraint: Constraint, params: Sequence[params]) -> str

to:

(err: ConstraintViolated) -> str

This was possible without making it less powerful by enriching ConstraintViolated, which now takes all three arguments that ErrorRephraser took separately: ctx, constraint and params. (breaking change)

This also means that an ErrorRephraser can now easily reuse the original error, which is useful if all you want is to add extra info before or after the original error! Example:

mutually_exclusive.rephrased(
    error=lambda err: f"{err}.\nUse --renderer, the other two options are deprecated."
)(
    option(
        "--renderer",
        type=click.Choice(["cairo", "opengl", "webgl"], case_sensitive=False),
        help="Select a renderer for your Scene.",
    ),
    option(
        "--use_opengl_renderer", is_flag=True, hidden=True,
        help="(Deprecated) Use --renderer=opengl.",
    ),
    option(
        "--use_cairo_renderer", is_flag=True, hidden=True,
        help="(Deprecated) Use --renderer=cairo.",
    ),
)

The original message is also available as format string field {error}. So the following is equivalent to the above code:

mutually_exclusive.rephrased(
    error="{error}.\nUse --renderer, the other two options are deprecated."
)(...)

[codecov-commenter]

Codecov Report

Merging #54 (49dddac) into master (1599195) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #54   +/-   ##
=======================================
  Coverage   96.28%   96.29%           
=======================================
  Files          20       20           
  Lines        1291     1294    +3     
=======================================
+ Hits         1243     1246    +3     
  Misses         48       48           

Impacted Files	Coverage Δ
cloup/constraints/__init__.py	100.00% <100.00%> (ø)
cloup/constraints/_conditional.py	97.77% <100.00%> (ø)
cloup/constraints/_core.py	96.09% <100.00%> (ø)
cloup/constraints/exceptions.py	100.00% <100.00%> (ø)

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