Improve dotnet-user-secret error message when project path is invalid

[natemcmaster]

From @dougbu on May 12, 2016 17:16

The error message displayed when {path} is invalid is not descriptive and references an argument rather than the command-line option.

For example,

dotnet user-secrets list --project \garbage

Results in

critical: Command failed : The directory name C:\garbage\ is invalid.
Parameter name: path

Note the parameter name mentioned above is inconsistent w/ the in #77. It's the same command-line option and the parameter should also be the same.

Copied from original issue: aspnet/UserSecrets#78

[natemcmaster]

From @muratg on May 26, 2016 6:18

Fit & finish bug. Moving to 1.0.1 for now.