Fix CS0168 and CS0219 warnings

[harshavardhana]

Build FAILED.
Warnings:
/home/travis/build/minio/minio-dotnet/Minio.Client.sln (default targets) ->
(Build target) ->
/home/travis/build/minio/minio-dotnet/Minio.Client/Minio.Client.csproj (default targets) ->
/usr/lib/mono/xbuild/12.0/bin/Microsoft.CSharp.targets (CoreCompile target) ->
    Client.cs(322,50): warning CS0168: The variable `ex' is declared but never used
    Client.cs(676,17): warning CS0219: The variable `queries' is assigned but its value is never used
Errors:
/home/travis/build/minio/minio-dotnet/Minio.Client.sln (default targets) ->
(Build target) ->
/home/travis/build/minio/minio-dotnet/Minio.Client/Minio.Client.csproj (default targets) ->
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Common.targets (_CopyAppConfigFile target) ->
    /usr/lib/mono/xbuild/12.0/bin/Microsoft.Common.targets: error : Cannot copy /home/travis/build/minio/minio-dotnet/Minio.Client/App.config to /home/travis/build/minio/minio-dotnet/Minio.Client/bin/Release/Minio.Client.dll.config, as the source file doesn't exist.
     2 Warning(s)
     1 Error(s)