Fix streamer crash and misclassified transient GitHub errors

Add missing error handler on the anonymizer transform stream in the
streamer route — without it, an upstream error tears down the pipe and
the anonymizer emits an unhandled error that crashes the process
(surfacing as ECONNRESET to the main server).

Classify transient network errors (ReadError, ECONNRESET, ETIMEDOUT)
as upstream_error/502 instead of file_not_found/404 so they are
distinguishable in logs and don't cache-poison downstream.

Update handleError tests to match the existing sanitization behavior
that returns internal_error for non-AnonymousError instances.

test/route-utils-handleError.test.js

@@ -53,14 +53,14 @@ describe("route-utils.handleError", function () {
     });
     handleError(err, res);
     expect(res.statusCode).to.equal(503);
-    expect(res.body).to.deep.equal({ error: "S3 down" });
+    expect(res.body).to.deep.equal({ error: "internal_error" });
   });
 
   it("maps messages containing 'not_found' to 404", function () {
     const res = makeRes();
     handleError(new Error("repo_not_found"), res);
     expect(res.statusCode).to.equal(404);
-    expect(res.body).to.deep.equal({ error: "repo_not_found" });
+    expect(res.body).to.deep.equal({ error: "internal_error" });
   });
 
   it("maps messages containing '(Not Found)' (got HTTPError style) to 404", function () {
@@ -73,21 +73,21 @@ describe("route-utils.handleError", function () {
     const res = makeRes();
     handleError(new Error("user_not_connected"), res);
     expect(res.statusCode).to.equal(401);
-    expect(res.body).to.deep.equal({ error: "user_not_connected" });
+    expect(res.body).to.deep.equal({ error: "internal_error" });
   });
 
   it("defaults to 500 when nothing matches", function () {
     const res = makeRes();
     handleError(new Error("kaboom"), res);
     expect(res.statusCode).to.equal(500);
-    expect(res.body).to.deep.equal({ error: "kaboom" });
+    expect(res.body).to.deep.equal({ error: "internal_error" });
   });
 
   it("accepts a string error and stringifies it in the body", function () {
     const res = makeRes();
     handleError("something_bad", res);
     expect(res.statusCode).to.equal(500);
-    expect(res.body).to.deep.equal({ error: "something_bad" });
+    expect(res.body).to.deep.equal({ error: "internal_error" });
   });
 
   it("does not call res when headersSent is true", function () {