Add opt-in AlphaESSApiError so callers can detect API failures

[Poshy163]

The write endpoints answer with data: null whether they succeed or fail, and api_post returns
json_response["data"] on success and None on failure. So a successful write looks identical to
a rejected one and callers are left guessing.

This adds raise_on_error, off by default. Leave it alone and nothing changes: API-level failures
log and return None as always. Turn it on and they raise AlphaESSApiError with code, msg,
expMsg, description and path attached, so success becomes "nothing was raised" instead of a
return value nobody can interpret.

client = alphaess(appID, appSecret, raise_on_error=True)
try:
    await client.setTimeChargeBySn(sysSn, 0, charge_list, discharge_list)
except AlphaESSApiError as err:
    print(err.code, err.expMsg)   # 6001 "time list is null"

Backwards compatibility

• no return type or value changes anywhere
• raise_on_error goes last in __init__, so positional callers are unaffected
• transport errors propagate exactly as before
• the existing 34 tests pass untouched, and homeassistant-alphaESS's 342 pass against this build
  without opting in

getdata() stays best-effort with the flag on — a refused endpoint leaves that key None rather
than costing the whole poll, since getTimeChargeBySn answers 6017 on most systems.

Bumped to 0.0.21.

Doc corrections, checked against the live API

• An empty period list is rejected with 6001 "time list is null" — Document the full API, add periodic charge/discharge endpoints, fix error handling #28 said [] was fine,
  which was wrong. Omitting a list gives 10001, outside the 6xxx range and not in the portal's
  table.
• Parameters are validated before permissions, so an early 6001 doesn't mean your account has
  access — you just never got far enough to find out.
• expMsg is often the only field naming the bad parameter, and was neither logged nor exposed.
• There are 21 endpoints in the portal, not 19. getMeterOffsetConfigInfo and
  updateMeterOffsetConfigInfo are scoped to the commercial & industrial document, so a standard
  account can't reach them. Documented, not implemented. The portal lists the getter as POST; it's
  GET, and POST returns 405.

[Poshy163]

@CharlesGillanders bump, also needs a release so CharlesGillanders/homeassistant-alphaESS#270 can be finished off