Support non-200 response status codes

These are used by a few API modules, notably some WikiLambda APIs (or
Wikifunctions APIs, as they’re apparently starting to be called now). I
hope that checking for the MediaWiki-API-Error response header is enough
to distinguish between such responses and genuine internal errors that
we shouldn’t even try to decode.

Only unit-tested, because this feature is so rare that none of the APIs
using it seem suitable for an integration test against a real MediaWiki.

Fixes #30.

CHANGES.md

@@ -7,6 +7,11 @@ but this file may sometimes contain later improvements (e.g. typo fixes).
 
 ## next (not yet released)
 
+- Error responses with non-200 HTTP status codes are now supported,
+  as long as the `MediaWiki-API-Error` response header is present
+  to indicate that the response represents a regular error and not an internal error.
+  (The status code is ignored;
+  the response body is expected to contain a response with one or more regular errors.)
 - Updated dependencies.
 
 ## v0.8.1 (2023-11-12)

core.js

@@ -557,7 +557,7 @@ class Session {
 			body,
 		} = internalResponse;
 
-		if ( status !== 200 ) {
+		if ( status !== 200 && !( 'mediawiki-api-error' in responseHeaders ) ) {
 			throw new Error( `API request returned non-200 HTTP status code: ${ status }` );
 		}
 

test/unit/core.test.js

@@ -86,7 +86,17 @@ describe( 'Session', () => {
 			await session.request( params );
 		} );
 
-		it( 'throws on non-200 status', async () => {
+		it( 'supports non-200 status with mediawiki-api-error response header', async () => {
+			const session = singleRequestSession( { action: 'query' }, {
+				status: 404,
+				headers: { 'mediawiki-api-error': 'some-not-found-error' },
+				body: { error: { code: 'some-not-found-error' } },
+			} );
+			await expect( session.request( { action: 'query' } ) )
+				.to.be.rejectedWith( ApiErrors );
+		} );
+
+		it( 'throws on non-200 status without mediawiki-api-error response header', async () => {
 			const session = singleRequestSession( { action: 'query' }, {
 				status: 502,
 				body: 'irrelevant',