darc vmr cherry-pick does not work when not run from VMR, even with --vmr

[mmitche]

When attempting to cherry-pick from a directory to the VMR, it is not obvious that the command must be run from the VMR directory. Failure to do so will yield a somewhat cryptic error like:

fail: Cherry-pick operation failed: Could not find file 'C:\r\runtime\src\source-mappings.json'.
dbug: System.IO.FileNotFoundException: Could not find file 'C:\r\runtime\src\source-mappings.json'.

This doesn't say what it actually means: That what it thinks it's cherry-picking to is not a VMR. The error messages in this case should be very clear, and include hints. the tool has detected that the cherry-picking operation is "starting from the VMR". It should be able to tell the user that the target (current directory is not a VMR). Better yet, it would be good for the command to say something like:

info: Cherry-picking <commit> from repository (<source>) -> VMR (<expected vmr dir>)
info: Detected source mapping name: runtime
fail: Cherry-pick operation failed: Could not find file 'C:\r\runtime\src\source-mappings.json'. Are you sure that <expected vmr dir> is the VMR? Operation should be run from VMR dir or with --vmr

Furthermore, passing --vmr does not work. For example:

e C:\r\runtime --commit 19e7456131cad3ad17cd16871f928d06e7e1b391
info: Cherry-pick operation starting from repository
info: Detected mapping name: runtime
fail: Cherry-pick operation failed: Could not find file 'C:\r\runtime\src\source-mappings.json'.
dbug: System.IO.FileNotFoundException: Could not find file 'C:\r\runtime\src\source-mappings.json'.
      File name: 'C:\r\runtime\src\source-mappings.json'
         at Microsoft.Win32.SafeHandles.SafeFileHandle.CreateFile(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options)
         at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
         at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
         at System.IO.Strategies.FileStreamHelpers.ChooseStrategyCore(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
         at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, Int64 preallocationSize)
         at System.IO.File.AsyncStreamReader(String path, Encoding encoding)
         at System.IO.File.InternalReadAllTextAsync(String path, Encoding encoding, CancellationToken cancellationToken)
         at Microsoft.DotNet.DarcLib.VirtualMonoRepo.SourceMappingParser.ParseMappings(String mappingFilePath) in /_/src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/SourceMappingParser.cs:line 40
         at Microsoft.DotNet.Darc.Operations.VirtualMonoRepo.CherryPickOperation.ExecuteInternalAsync() in /_/src/Microsoft.DotNet.Darc/Darc/Operations/VirtualMonoRepo/CherryPickOperation.cs:line 84
         at Microsoft.DotNet.Darc.Operations.VirtualMonoRepo.CherryPickOperation.ExecuteAsync() in /_/src/Microsoft.DotNet.Darc/Darc/Operations/VirtualMonoRepo/CherryPickOperation.cs:line 49

This needs to be fixed.