minor bugfixes for indexcov and indexsplit

v0.2.0

• indexcov: better error message and handling of excluded chromosomes.
• indexcov: if given crais and no fai, indexcov will try to read the cram header using samtools view.
• indexsplit: fix rare panic in CRAI files due to an off-by-one error (thanks Lavanya for reporting and providing a test-case).