Better error catching for Dada

[gregdenay]

So far any error raised in the Dada2 R script will result in blank output.
This is needed to handle NTC samples where no reads cluster but it also makes debugging and problem finding harder and should be corrected.

Implement proper error catching and handling (as far as R allows it) in the script.
The analysis should go on if a sample dose not lead to cluster formation (no/few reads, bad quality) and pipeline should stop on error for other cases.

[gregdenay]

fixed in #39