Handle multiple validation reports in error(). Closes #1940

API.md

@@ -838,7 +838,8 @@ schema.validate(''); // returns { error: "value" is not allowed to be empty, val
 Overrides the default **joi** error with a custom error if the rule fails where:
 - `err` can be:
   - an instance of `Error` - the override error.
-  - a function with the signature `function(errors)`, where `errors` is an array of errors and it returns a single `Error`.
+  - a function with the signature `function(errors)`, where `errors` is an array of validation
+    reports and it returns either a single `Error` or an array of validation reports.
 
 Note that if you provide an `Error`, it will be returned as-is, unmodified and undecorated with any
 of the normal error properties. If validation fails and another error is found before the error

lib/validator.js

@@ -249,7 +249,15 @@ internals.finalize = function (value, schema, original, errors, state, prefs) {
     if (errors.length &&
         schema._flags.error) {
 
-        errors = [typeof schema._flags.error === 'function' ? schema._flags.error(errors) : schema._flags.error];
+        if (typeof schema._flags.error === 'function') {
+            errors = schema._flags.error(errors);
+            if (!Array.isArray(errors)) {
+                errors = [errors];
+            }
+        }
+        else {
+            errors = [schema._flags.error];
+        }
     }
 
     // Default