Use bool instead of boolean while throwing a type error

Use bool instead of boolean while throwing a type error

PHP requires boolean typehints to be written "bool" and disallows
"boolean" as an alias. This changes the error messages to match
the actual type name and avoids confusing messages like "must be
of type boolean, boolean given".

This a followup to ce1d69a1f6dcf15d43029301059c25e5bc09a577, which
implements the same change for integer->int.