Allow changing server if validation has failed

Show the server config section if there's an error and fix an if
case where we forgot to un-set the busy flag

src/components/structures/auth/Registration.js

@@ -168,6 +168,8 @@ module.exports = React.createClass({
     _replaceClient: async function(serverConfig) {
         this.setState({
             errorText: null,
+            serverDeadError: null,
+            serverErrorIsFatal: false,
             // busy while we do liveness check (we need to avoid trying to render
             // the UI auth component while we don't have a matrix client)
             busy: true,
@@ -429,7 +431,9 @@ module.exports = React.createClass({
 
         // If we're on a different phase, we only show the server type selector,
         // which is always shown if we allow custom URLs at all.
-        if (PHASES_ENABLED && this.state.phase !== PHASE_SERVER_DETAILS) {
+        // (if there's a fatal server error, we need to show the full server
+        // config as the user may need to change servers to resolve the error).
+        if (PHASES_ENABLED && this.state.phase !== PHASE_SERVER_DETAILS && !this.state.serverErrorIsFatal) {
             return <div>
                 <ServerTypeSelector
                     selected={this.state.serverType}

src/components/views/auth/ServerConfig.js

@@ -104,6 +104,9 @@ export default class ServerConfig extends React.PureComponent {
 
             const stateForError = AutoDiscoveryUtils.authComponentStateForError(e);
             if (!stateForError.isFatalError) {
+                this.setState({
+                    busy: false,
+                });
                 // carry on anyway
                 const result = await AutoDiscoveryUtils.validateServerConfigWithStaticUrls(hsUrl, isUrl, true);
                 this.props.onServerConfigChange(result);