Attempt to fix #82

[carstenbauer]

Closes #82

I use try/catch to make the tasks that don't get any elements from the channel return nothing gracefully. Afterwards, I filter out these results before performing the final reduction.

Disadvantages:

• We potentially ignore MethodErrors cause by user input (f, `op).
• results = fetch.(tasks) allocates

[carstenbauer]

Closed in favor of #84