Adds error handling to configuration request

Catch request exception in case fetching the configuration from the url fails, this can happen either if non existent version number is passed in or because of server issues.

Author: aszenz

docs/index.html

@@ -100,16 +100,20 @@
                   if (this.version !== this.oldVersion) {
                     const ConfigurationMdUrl =
                       `https://raw.githubusercontent.com/rust-lang/rustfmt/${this.version}/Configurations.md`;
-                    const res = await axios.get(ConfigurationMdUrl);
-                    const {
-                      about,
-                      configurationAbout,
-                      configurationDescriptions
-                    } = parseMarkdownAst(res.data);
-                    this.aboutHtml = marked.parser(about);
-                    this.configurationAboutHtml = marked.parser(configurationAbout);
-                    this.configurationDescriptions = configurationDescriptions;
-                    this.oldVersion = this.version;
+                    try {
+                      const res = await axios.get(ConfigurationMdUrl);
+                      const {
+                        about,
+                        configurationAbout,
+                        configurationDescriptions
+                      } = parseMarkdownAst(res.data);
+                      this.aboutHtml = marked.parser(about);
+                      this.configurationAboutHtml = marked.parser(configurationAbout);
+                      this.configurationDescriptions = configurationDescriptions;
+                      this.oldVersion = this.version;
+                    } catch(error) {
+                        this.aboutHtml = "<p>Failed to get configuration options for this version, please select the version from the dropdown above.</p>";
+                    }
                   }
 
                   const ast = this.configurationDescriptions