fix: show custom error messages when hide_input=True

[MAXDVVV]

Problem

When using hide_input=True with a custom type that raises BadParameter (via self.fail()), the custom error message is swallowed and replaced with a generic "Error: The value you entered was invalid.".

This is because termui.py has an explicit branch that replaces any UsageError message with a hardcoded string when hide_input=True:

except UsageError as e:
    if hide_input:
        echo("Error: The value you entered was invalid.")  # custom message lost!
    else:
        echo(f"Error: {e.message}")  # custom message shown

Reproduction

class StrictPassword(click.ParamType):
    name = "password"
    def convert(self, value, param, ctx):
        if len(value) < 8:
            self.fail("Password must be at least 8 characters", param, ctx)
        return value

@click.command()
@click.option("--pw", prompt=True, hide_input=True, type=StrictPassword())
def cmd(pw): ...

Entering "short" shows: Error: The value you entered was invalid.
Expected: Error: Password must be at least 8 characters

Fix

Remove the hide_input branch in the UsageError handler — always show the actual error message. The hide_input flag should only affect whether the input value is echoed during typing, not whether the developer's custom validation error messages are displayed.

Custom error messages are written by the developer and don't contain the secret input value, so there's no security reason to suppress them.

Test

Added test_hide_input_shows_custom_error — verifies custom error messages from type validation are shown even when hide_input=True.

Fixes #2809