Improve error messages on add new server dialog

Closes #438

Note: this solution is different than what @jasonblais suggested.
@jasonblais suggested not highlighting the field that hasn't been
filled out yet, but I think it's a better UX to continue to highlight
them all as it currently does, but to clarify the error messages. Also
notably, as far as I can tell, the react-bootstrap [forms
API](https://react-bootstrap.github.io/components/forms/) does not
appear to support selectively highlighting some fields in the form but
not others anyway.

CHANGELOG.md

@@ -50,6 +50,8 @@ Release date: TBD
 [#707](https://github.com/mattermost/desktop/issues/707)
 - Fixed an issue that prevented typing in the form fields on the add server dialog when launched from the tab bar.
 [#780](https://github.com/mattermost/desktop/issues/780)
+- Fixed an issue that could cause an error message on the add new server dialog to be misleading.
+[#438](https://github.com/mattermost/desktop/issues/438)
 
 #### Windows
 - Fixed `file://` protocol not working. But localhost URL will not continue to work.

src/browser/components/NewTeamModal.jsx

@@ -68,7 +68,17 @@ export default class NewTeamModal extends React.Component {
   }
 
   getError() {
-    return this.getTeamNameValidationError() || this.getTeamUrlValidationError();
+    const nameError = this.getTeamNameValidationError();
+    const urlError = this.getTeamUrlValidationError();
+
+    if (nameError && urlError) {
+      return 'Name and URL are required.';
+    } else if (nameError) {
+      return nameError;
+    } else if (urlError) {
+      return urlError;
+    }
+    return null;
   }
 
   validateForm() {