The current API build scripts will continue and exit with a zero
status even if they find problems. This has been the cause of many
build problems, or hidden build errors, in the past. Change the
scripts so they always exit with a non-zero status for any problems
they do not understand. Also turn off all debug output by default
so they respect $(AM_V_GEN)

* docs/Makefile.am: Use $(AM_V_GEN) for API/HTML scripts
* docs/apibuild.py, python/generator.py: Exit with non-zero status
  if problems are found. Also be silent, not outputting any debug
  messages.
* src/Makefile.am: Use $(AM_V_GEN) for ESX generator
* python/Makefile.am: Tweak rule