Improve HTTP error text (#364)

Thanks to Craig for the suggestion! Since people may have, before #363,
been parsing the text, I mentioned it as a breaking change, although I
imagine most people are parsing just the code, not the text.

I have:
- [x] Written a clear PR title and description (above)
- [x] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
- [x] Added tests covering my changes, if applicable
- [x] Included a link to the issue fixed, if applicable
- [x] Included documentation, for new features
- [x] Added an entry to the changelog

docs/CHANGELOG.md

@@ -26,6 +26,7 @@ Note that genqlient now requires Go 1.22.5 or higher, and is tested through Go 1
 
 - omitempty validation:
   - forbid `omitempty: false` (including implicit behaviour) when using pointer on non-null input field
+- The error text for HTTP errors has changed slightly. If you were parsing it, switch to [`As`-ing to `graphql.HTTPError`](client_config.md#handling-errors)_.
 
 ### New features:
 

graphql/errors.go

@@ -10,5 +10,5 @@ type HTTPError struct {
 
 // Error implements the error interface for HTTPError.
 func (e *HTTPError) Error() string {
-	return fmt.Sprintf("returned error %v: %s", e.StatusCode, e.Body)
+	return fmt.Sprintf("returned error %v: '%s'", e.StatusCode, e.Body)
 }