This avoids the awkward triple return state (NO_SUCH_GPROPERTY,
VALUE_WAS_SET, SOME_ERROR_OCCURRED) by splitting init_g_param_from_prop()
into an infallible function that looks up the GParamSpec (returning null
if no such GObject property) and a function that initializes the
GParameter from the GParamSpec (returning false on error).

Moves the check for overridden JS properties to the place where they are
set while not constructing, since we previously passed a bool in two
places in the code for that.

See: #160