[lldb][Test] Propagate llvm::yaml error message in TestFile::fromYaml

Currently the test-suite would swallow the error message
on `llvm::yaml::convertYAML` failures.

This patch simply propagates the error string up to the caller.

Before patch:
```
[ RUN      ] DWARFASTParserClangTests.TestDefaultTemplateParamParsing
/Users/michaelbuch/Git/llvm-worktrees/playground/lldb/unittests/TestingSupport/Symbol/YAMLModuleTester.cpp:19: Failure
Value of: llvm::detail::TakeExpected(File)
Expected: succeeded
  Actual: failed  (convertYAML() failed: )
Assertion failed: (!HasError && "Cannot get value when an error exists!"), function getStorage, file Error.h, line 671.
```

After patch:
```
[ RUN      ] DWARFASTParserClangTests.TestDefaultTemplateParamParsing
/Users/michaelbuch/Git/llvm-worktrees/playground/lldb/unittests/TestingSupport/Symbol/YAMLModuleTester.cpp:19: Failure
Value of: llvm::detail::TakeExpected(File)
Expected: succeeded
  Actual: failed  (convertYAML() failed: wrote too much data somewhere, section offsets don't line up)
Assertion failed: (!HasError && "Cannot get value when an error exists!"), function getStorage, file Error.h, line 671.
```

Differential Revision: https://reviews.llvm.org/D140112