Warn if adapters cannot be indexed

[marcelm]

When providing lots of 5' anchored adapters with -e 2, an index can be generated and finding the adapters (barcodes) is fast. When then the maximum allowed number of errors is increased (say with -e 3), no index is generated (because an index is only generated up to two allowed errors), which has two consequences:

• Matching becomes slow
• Possibly ambiguous adapter/barcode assignments are no longer detected

We cannot always warn when no index can be created, but maybe when there are at least a certain number of anchored sequences and only the error rate is too high.

CC @simonelsasser
CC @cnluzon