Improve input validation for FASTQ

[scwatts]

Description of feature

Validation of input FASTQ files should be improved:

• certain unsupported samplesheet FASTQ input rows produce non-descript errors
  • see here for an example: https://nfcore.slack.com/archives/C04CQ1LK7U1/p1724454431405079
• all file inputs including FASTQ should be init as Nextflow.File objects and checked to exist (w/ stub mode exception):

https://github.com/nf-core/oncoanalyser/blob/4227bc1083209d79c8735d32e6c75f762a640006/lib/Utils.groovy#L127-L133

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This work may also relate to:

• https://github.com/nf-core/oncoanalyser/issues/51

[scwatts]

Additionally where a samplesheet has FASTQ input files provided as separate entries rather than the expected single entry, an informative error should be provided. See: https://nfcore.slack.com/archives/C04CQ1LK7U1/p1734010565913889