Revert "wfe: on rate limit error, serve 500 (#7796)"

This reverts commit 242d746.

We want to make this change, but it carries some risk that we'd prefer not to
take over the holiday. We'd also like to keep `main` in a state where it would
be reasonable to deploy (even if, in practice, any over-the-holiday deploy would
be a hotfix, not a direct tag from `main`).

web/context.go

@@ -35,7 +35,12 @@ type RequestEvent struct {
 	Slug           string   `json:",omitempty"`
 	InternalErrors []string `json:",omitempty"`
 	Error          string   `json:",omitempty"`
-	UserAgent      string   `json:"ua,omitempty"`
+	// If there is an error checking the data store for our rate limits
+	// we ignore it, but attach the error to the log event for analysis.
+	// TODO(#7796): Treat errors from the rate limit system as normal
+	// errors and put them into InternalErrors.
+	IgnoredRateLimitError string `json:",omitempty"`
+	UserAgent             string `json:"ua,omitempty"`
 	// Origin is sent by the browser from XHR-based clients.
 	Origin string                 `json:",omitempty"`
 	Extra  map[string]interface{} `json:",omitempty"`

wfe2/wfe.go

@@ -798,8 +798,7 @@ func (wfe *WebFrontEndImpl) NewAccount(
 			wfe.sendError(response, logEvent, probs.RateLimited(err.Error()), err)
 			return
 		} else {
-			wfe.sendError(response, logEvent, web.ProblemDetailsForError(err, "While checking rate limits"), err)
-			return
+			logEvent.IgnoredRateLimitError = err.Error()
 		}
 	}
 
@@ -2407,13 +2406,14 @@ func (wfe *WebFrontEndImpl) NewOrder(
 	}
 
 	refundLimits, err := wfe.checkNewOrderLimits(ctx, acct.ID, names, isRenewal)
-	if err != nil && features.Get().UseKvLimitsForNewOrder {
+	if err != nil {
 		if errors.Is(err, berrors.RateLimit) {
-			wfe.sendError(response, logEvent, probs.RateLimited(err.Error()), err)
-			return
+			if features.Get().UseKvLimitsForNewOrder {
+				wfe.sendError(response, logEvent, probs.RateLimited(err.Error()), err)
+				return
+			}
 		} else {
-			wfe.sendError(response, logEvent, web.ProblemDetailsForError(err, "While checking rate limits"), err)
-			return
+			logEvent.IgnoredRateLimitError = err.Error()
 		}
 	}