oe-init-build-env: Error out when failed to locate cwd

oe-init-build-env: Error out when failed to locate cwd

Ubuntu's /bin/sh symlinks to /bin/dash by default, so
subprocess.check_call(oe-init-build-env, cwd=builddir) would be failed since
pwd is builddir, and there is no $builddir/oe-init-build-env, this would
lead to other confusing errors, check and error it out earlier to make it
easier to locate the problem.

We don't meet the problem when manually run ". oe-init-build-env" is because
Ubuntu's default login shell is bash, but subprocess.check_call() doesn't
respect to login shell, so the error only happens in situations like
subprocess.check_call().

And also print errors to stderr as oe-buildenv-internal does.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>