[pdb2mdb] Detect portable PDB and fail gracefully instead of crashing (#4261)

[pdb2mdb] Detect portable PDB and fail gracefully instead of crashing (#4261)

Before when trying to convert an assembly that had a portable PDB
instead of a native PDB alongside it we'd just crash in the
Microsoft.Cci pdb reader.

We now check whether the symbol file is a portable PDB and exit
gracefully with an error message and exit code 2 from the app.

Users which access the Converter class (like e.g. Xamarin Android [1])
directly can now catch the PortablePdbNotSupportedException to
do custom logic.

[1] https://github.com/xamarin/xamarin-android/blob/15f7547fa673e02256a75910c60c5e13eb158d2b/src/Xamarin.Android.Build.Tasks/Tasks/ConvertDebuggingFiles.cs#L28