Reduce use of abort, remove assert(fp), pass one string to fprintf not multicall

Reduce use of abort to situations where you might actually want to debug.
Remove assert(fp)'s and replace with proper error messages.
Remove a few instances where fprintf is called multiple times rather than just
passing one string to it.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Recanonicalise padding.c whitespace -- USE -b/-w TO DIFF/BLAME

This file had a mixture of 1-tab and 8-space indents which previously
went unnoticed.  See notes in fcb8af08a6fd3bfcf195407c4ba5bae528ee326d.
Merge abort/assert reduction (PR #290)

Also update bam_stat.c and bamshuf.c dependencies accordingly.