KPluginMetaData: warn if library metadata can't be loaded

I was debugging why the KIO unit tests were failing on my CHERI-RISC-V QEMU instance. For some reason the kf5/kio/file.so was not being loaded but there was no warning output. Turns out that I originally added warning messages for invalid .desktop and .json files back in 2014 but I forgot to add the qCWarning() for shared libraries.

With this change I got a useful warning message that indicated that I had a corrupted file in my build directory: kf.coreaddons: No metadata found in "/build/bin/kf5/kio/file.so" "'/build/bin/kf5/kio/file.so' is an invalid ELF object (shstrtab section header seems to be at 1233d0)"