Mention header name error message for invalid header encodings

[RobertCraigie]

This openai-python user ran into a confusing error when passing a non-ascii header value, would it be possible to mention the header name in the error message?

Minimal repro

import httpx

httpx.Headers({"auth": "здравейздравейздравейздравей"})

Traceback (most recent call last):
  File "script.py", line 3, in <module>
    httpx.Headers({"auth": "здравейздравейздравейздравей"})
  File ".venv/lib/python3.9/site-packages/httpx/_models.py", line 74, in __init__
    self._list = [
  File ".venv/lib/python3.9/site-packages/httpx/_models.py", line 78, in <listcomp>
    normalize_header_value(v, encoding),
  File ".venv/lib/python3.9/site-packages/httpx/_utils.py", line 53, in normalize_header_value
    return value.encode(encoding or "ascii")
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-27: ordinal not in range(128)

[tomchristie]

That'd be a fantastic idea yep. What do we want the exception message to look like here?

[RobertCraigie]

Maybe something like this?

httpx.HeaderEncodeError: 'ascii' codec cannot encode the 'HeaderName' header with 'value'

not sure if it should be a header specific error or more generic like httpx.EncodeError?

[tomchristie]

ValueError would be fine.