[fix] Wrap calls to format.transform with try / catch. Potential fi…

…x for #1261.

lib/winston/logger.js

@@ -216,9 +216,17 @@ class Logger extends stream.Transform {
     }
 
     // Here we write to the `format` pipe-chain, which on `readable` above will
-    // push the formatted `info` Object onto the buffer for this instance.
-    this.push(this.format.transform(info, this.format.options));
-    callback();
+    // push the formatted `info` Object onto the buffer for this instance. We trap
+    // (and re-throw) any errors generated by the user-provided format, but also
+    // guarantee that the streams callback is invoked so that we can continue flowing.
+    try {
+      this.push(this.format.transform(info, this.format.options));
+    } catch (ex) {
+      throw ex;
+    } finally {
+      // eslint-disable-next-line callback-return
+      callback();
+    }
   }
 
   /**