…cho in the generator

This commit fixes pallets#2674 (echo_via_pager with generators leaves terminal in broken state).

When running the program to reproduce the issue (thanks to @0xDEC0DE for providing it in the issue mentioned above!)

```
import click

def output_generator():
    counter = 0
    while True:
        yield "this is a line of output\n"
        if counter == 1024:
            click.echo("kaboom", err=True)
            click.get_current_context().exit(0)
        counter += 1

@click.command
def kaboom():
    click.echo_via_pager(output_generator)

if __name__ == "__main__":
    kaboom()
```

The "kaboom" message will now be visible immediately (the error will cause click to terminate the pager). ZSH and bash will not be in a broken state (tested using bash and zsh on macos).

Further changes:
- Error handling: terminate the pager and close the file descriptors in a finally block
- KeyboardInterrupt: the pager code will not ignore KeyboardInterrupt anymore. This allows the user to
  search for future output of the generator when using less and then aborting the program using ctrl-c.
- tests: improved the echo_via_pager tests by using a named tuple and separate assertions for pager
  output, stderr and stdout. Added additional test cases.