Makefile: should crash when finding N>2 fastq files

[e-t-k]

Weakness in current Makefile: as written will happily process the first R1 and R2 fastqs that it finds, ignoring any additional fastqs in dir that are found by the find command. This potentially results in a sample processed using only a subset of its reads, with no indication that anything is wrong.

If there are more than 2 valid fastqs, Makefile should instead crash. (Ideally, process them all; but that is not priority to implement as fab can take care of it.)

also minor cleanup:

• Fix this echo - this target actually only runs expression, not QC
  https://github.com/UCSC-Treehouse/pipelines/blob/master/Makefile#L41