Sam-merge error checking doesn't work well with BWA aligned reads

BWA's output bam generates some warning which in turn cause sam-merge to fail:
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Ignoring SAM validation error: ERROR: Record 12684017, Read name D00539:104:H5T7GBCXX:2:2216:1236:23192, MAPQ should be 0 for unmapped read.
Ignoring SAM validation error: ERROR: Record 12401587, Read name D00539:104:H5T7GBCXX:1:1102:7867:12491, MAPQ should be 0 for unmapped read.
Ignoring SAM validation error: ERROR: Record 12685834, Read name D00539:104:H5T7GBCXX:2:2216:1236:23192, MAPQ should be 0 for unmapped read.
Ignoring SAM validation error: ERROR: Record 12685835, Read name D00539:104:H5T7GBCXX:2:1112:4919:44036, MAPQ should be 0 for unmapped read.
Ignoring SAM validation error: ERROR: Record 12685836, Read name D00539:104:H5T7GBCXX:2:2208:18665:39296, MAPQ should be 0 for unmapped read.
Ignoring SAM validation error: ERROR: Record 12685837, Read name D00539:104:H5T7GBCXX:2:2203:14340:2714, MAPQ should be 0 for unmapped read.
Ignoring SAM validation error: ERROR: Record 12685838, Read name D00539:104:H5T7GBCXX:2:2113:19252:76901, MAPQ should be 0 for unmapped read.
## Ignoring SAM validation error: ERROR: Record 12685839, Read name D00539:104:H5T7GBCXX:2:1112:1458:9142, MAPQ should be 0 for unmapped read.

For sam-merge to work with BWA, stdio regex should ignore SAM validation errors or an option should be provided to ignore these errors.

No file or test is identified. Start by locating sam-merge's stdio regex handling and how it processes BWA-generated BAM validation warnings; done means BWA's reported MAPQ validation warnings no longer make sam-merge fail, either through the regex or an explicit ignore option.

Written by the indexing model from the issue text.