Fix #1460: hide prompt input if hide_input is enabled

[csr]

This PR aims to fix #1460 (prompt input would be displayed in the error message even if hide_input was enabled).

These scripts now work just fine and display Error: the value you entered was invalid if the input isn't what Click expects. Script 1:

cesaredecal$ python  -c 'import click; click.prompt("prompt", type=int, hide_input=True)'
prompt: 
Error: the value you entered was invalid

Script 2 (that asks for a numeric pin):

import click


@click.command()
@click.password_option(type=int)
def cli(password):
    pass


cli()

cesaredecal$ python hello.py
Password: 
Error: the value you entered was invalid

All tests pass.

[davidism]

No tests were added because it was unclear how to test hidden prompts.