Better error reporting when a file is not found in the copy task

[gijsroge]

Is this something I can implement in the current state? Or are you planning on revising the entire copy functionality any time soon?

This is the current output when you have a file that cannot be found:

[14:29:43] Starting 'setEnvironment'...
[14:29:43] Finished 'setEnvironment' after 537 μs
[14:29:43] Starting 'clean'...
[14:29:43] Finished 'clean' after 3.21 ms
[14:29:43] Starting 'copy'...
[14:29:43] The following tasks did not complete: build, copy
[14:29:43] Did you forget to signal async completion?
error Command failed with exit code 1.