[lldb][expr] Propagate ClangDynamicCheckerFunctions::Install() errors to caller

[lldb][expr] Propagate ClangDynamicCheckerFunctions::Install() errors to caller

I came accross this, because a lot of regression tests were saying:
```
(lldb) p argc
error: expression failed to parse:
error: couldn't install checkers, unknown error
```

With this change, error messages provide more detail:
```
(lldb) p argc
error: expression failed to parse:
error: couldn't install checkers:
error: Couldn't lookup symbols:
  __objc_load
```

I didn't find a case where `Diagnostics()` is not empty. Also it looks like this isn't covered in any test (yet).

Reviewed By: bulbazord, Michael137

Differential Revision: https://reviews.llvm.org/D146541