Improve InvalidURL error message.

[tomchristie]

Prompted by #3248.

before...

>>> import httpx
>>> httpx.get("https://www.example.com\n")
Traceback (most recent call last):
  ...
  File "/Users/tomchristie/GitHub/encode/httpx/httpx/_urlparse.py", line 163, in urlparse
    raise InvalidURL(error)
httpx.InvalidURL: Invalid non-printable ASCII character in URL.

after...

>>> import httpx
>>> httpx.get("https://www.example.com\n")
Traceback (most recent call last):
  ...
  File "/Users/tomchristie/GitHub/encode/httpx/httpx/_urlparse.py", line 168, in urlparse
    raise InvalidURL(error)
httpx.InvalidURL: Invalid non-printable ASCII character in URL, '\n' at position 23.

[adriangb]

Minor nits