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.

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() {