If we fail due to a missing command (always bison)
during the build, it is running many things in parallel
and the error message gets lost in the noise.
Also diagnose bison++.

$ ./make.bash
Your system's bison is bison++, a buggy copy of the original bison.
Go needs the original bison instead.
See http://golang.org/doc/install.html#ctools
$ sudo apt-get remove bison++
... ridiculous amount of output ...
$ ./make.bash
Cannot find 'bison' on search path.
See http://golang.org/doc/install.html#ctools
$ sudo apt-get install bison
... ridiculous amount of output ...
$ ./make.bash
... works

Fixes #1938.
Fixes #1946.

R=bradfitz
CC=golang-dev
https://golang.org/cl/4528137