Output error message for pooled assembly of single-end reads

[kmnip]

Please report

• [x] version of RNA-Bloom with java -jar RNA-Bloom.jar -version 2.0.1
• [x] version of java with java -version 20
• [x] exact command used to run RNA-Bloom rnabloom -pool readslist.txt

If an input readslist file for the -pool option contains only single-end reads, stage 1 will still run to the end.

Technically, RNA-Bloom should exit with an error indicating "pooled assembly of single-end reads is not supported." I can also emphasize this (and explain why) in the readme section for -pool option: https://github.com/bcgsc/RNA-Bloom#file-format-for-the--pool-option