- converted `log_path` and `env_path` to properties of PackageBase.
- InstallErrors in build_environment are now annotated with the package
that caused them, in the 'pkg' attribute.
- Add `--show-log-on-error` option to `spack install` that catches
InstallErrors and prints the log to stderr if it exists.
Note that adding a reference to the Pakcage allows a lot of stuff
currently handled by do_install() and build_environment to be handled
externally.
c7a789e2d6