[clang] [clang][ASTImporter] Fix of unchecked Error object (NFC) (PR #112688)

[Balázs Kéri via cfe-commits]

balazske wrote:

I do not want now to check what is exactly happening, the case is a bit interesting. Normally `return Err` should work and is used at other functions too. And why did not a similar crash happen at the unit tests? Probably it is caused by different compilation on other platform or when "copy elision" does not happen. It looks difficult to tell if this is a NFC or not, if not there should be a test to reproduce the crash but tests for default template arguments exist already (at least the affected function should be called from some test).

https://github.com/llvm/llvm-project/pull/112688