fix(rests): Fix crash on undefined trait uses

Traits marked to recharge with no defined 'uses' attribute will no longer cause the script to crash.
An error will now sent to the log in this case.
fixes: #81

lib/rest-manager.js

@@ -118,7 +118,13 @@ class RestManager {
           this.logger.debug(`Recharging '${attName}'`);
           traitNames.push(attName);
           const max = this.roll20.getAttrByName(charId, `${traitPre}_uses`, 'max');
-          this.roll20.setAttrByName(charId, `${traitPre}_uses`, max);
+          if (max === undefined) {
+            this.logger.error(`Tried to recharge the trait '${attName}' for character with id ${charId}, ` +
+              'but there were no uses defined.');
+          }
+          else {
+            this.roll20.setAttrByName(charId, `${traitPre}_uses`, max);
+          }
         }
       });