readXStringSet uses non-canonical warnings

[ahl27]

Noting this for my own reference to fix in the (near) future:

readXStringSet internal methods do not use the correct method of throwing warnings. I'm assuming this is the case for both FASTA and FASTQ files. The error comes from read_fasta_files.c (and equivalent for fastq)--internal methods call snprintf rather than warning(). This means that warnings cannot be suppressed using R error handling (e.g., suppressWarnings) or elevated with options(warn=2). See for example read_fasta_files.c:326.