bootstrap: fix error reporting if the root is not valid

Autoproj.error requires a message argument
bootstrap: fix exception raised in validate_autoproj_current_root

The backtrace was previously displayed. This makes sure that only the
error message is
bootstrap: fix invalid setting of Autoproj.osdeps
osdeps: fix override-check in OSDependencies#merge()

The current code was wrongly injecting 'h' which is a package name
instead of osdep_h which is the hash being built. It would trigger
only if an osdep is overriden, which uses both a OS package and an
os-independent package.
released 1.12.4