Report validity state of all registration fields on any change

This passes the validity state of all fields to the consumer of
`RegistrationForm` via the `onValdiationChange` callback, instead of just the
most recent error.

In addition, we notify the consumer for any validation change, whether success
or failure. This allows old validation messages to be properly cleared. It also
allows the consumer to be aware of multiple validation errors and display the
next one after you have fixed the first.

Fixes https://github.com/vector-im/riot-web/issues/8769

src/components/structures/auth/Registration.js

@@ -288,7 +288,16 @@ module.exports = React.createClass({
         });
     },
 
-    onFormValidationFailed: function(errCode) {
+    onFormValidationChange: function(fieldErrors) {
+        // Find the first error and show that.
+        const errCode = Object.values(fieldErrors).find(value => value);
+        if (!errCode) {
+            this.setState({
+                errorText: null,
+            });
+            return;
+        }
+
         let errMsg;
         switch (errCode) {
             case "RegistrationForm.ERR_PASSWORD_MISSING":
@@ -490,7 +499,7 @@ module.exports = React.createClass({
                 defaultPhoneNumber={this.state.formVals.phoneNumber}
                 defaultPassword={this.state.formVals.password}
                 minPasswordLength={MIN_PASSWORD_LENGTH}
-                onError={this.onFormValidationFailed}
+                onValidationChange={this.onFormValidationChange}
                 onRegisterClick={this.onFormSubmit}
                 onEditServerDetailsClick={onEditServerDetailsClick}
                 flows={this.state.flows}