Fix mermaid error handling

Author: SISheogorath

public/js/extra.js

@@ -378,9 +378,14 @@ export function finishView (view) {
       $ele.html($value.text())
       window.mermaid.init(undefined, $ele)
     } catch (err) {
+      var errormessage = err
+      if (err.str) {
+        errormessage = err.str
+      }
+
       $value.unwrap()
-      $value.parent().append('<div class="alert alert-warning">' + err + '</div>')
-      console.warn(err)
+      $value.parent().append('<div class="alert alert-warning">' + errormessage + '</div>')
+      console.warn(errormessage)
     }
   })
   // abc.js