* fix: handle non-native Error throws in requirePnpmfile
When a pnpmfile throws a non-native Error value (e.g. a string),
`assert(util.types.isNativeError(err))` crashes pnpm with an
unhelpful assertion failure. Replace the assertion with a guard
that wraps non-native errors into a proper Error and reports them
via PnpmFileFailError.
* fix: improve non-native error wrapping with toError helper
