errors: don't fail on broken primary translations

errors: don't fail on broken primary translations

If a primary bundle doesn't contain a message then the fallback bundle
is used. However, if the primary bundle's message is broken (e.g. it
refers to a interpolated variable that the compiler isn't providing)
then this would just result in a compiler panic. While there aren't any
primary bundles right now, this is the type of issue that could come up
once translation is further along.

Signed-off-by: David Wood <david.wood@huawei.com>