[llvm-cov] - Output better error message when the error kind is...

[llvm-cov] - Output better error message when the error kind is `coveragemap_error::malforme`. (#65264)

The current llvm-cov error message for kind `coveragemap_error::malforme`, just gives the issue kind without any reason for what caused the issue. This patch is aimed at improving the llvm-cov error message to help identify what caused the issue.

Reviewed By: MaskRay
Close: https://github.com/llvm/llvm-project/pull/65264