Incremental tasks should show some information on triggering

[nightroman]

Currently, if an incremental task is skipped then it writes a message

Skipping up-to-date output.

Similarly, if an incremental task is triggered due to out of date of missing
outputs then it should write some information about those outputs.

A simple incremental task should tell, for example

Output X is missing or out-of-date.

A partial incremental task should tell, for example

N outputs of M inputs are missing or out-of-date.

Such messages may help on troubleshooting, e.g. see the case mentioned in #48.