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.
Fix one whitespace error.
Compare how this has been formatted in `sort_usage()` in bam_sort.c; putting the strings at the left margin means 80-column terminals have a good chance to display the source code for 80ish-column usage without wrapping.

Let's take this opportunity to exterminate the leading newline and the extra trailing newline.