fix: legible error on missing template placeholder (v0.2.1)

next() on a template source did template.format(...) directly, so a placeholder not
supplied in next(**fields) leaked a bare KeyError. catch KeyError/IndexError and
raise a clear ValueError naming the missing placeholder.

verified: next() without a required field -> ValueError naming it; happy path intact.
Signed-off-by: disqualifier <dev@disqualifier.me>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "aioproxies"
-version = "0.2.0"
+version = "0.2.1"
 description = "proxy parsing, formatting, health, and pool management for aiohttp/aioweb, camoufox, and socks5"
 requires-python = ">=3.10"
 dependencies = []