Don't use VstsCredProvider if build task env vars are set (#56)

Build.props

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
-    <CredentialProviderVersion>0.1.10</CredentialProviderVersion>
+    <CredentialProviderVersion>0.1.11</CredentialProviderVersion>
   </PropertyGroup>
 </Project>

CredentialProvider.Microsoft/CredentialProviders/Vsts/VstsCredentialProvider.cs

@@ -37,6 +37,18 @@ namespace NuGetCredentialProvider.CredentialProviders.Vsts
 
         public override async Task<bool> CanProvideCredentialsAsync(Uri uri)
         {
+            // If for any reason we reach this point and any of the three build task env vars are set, 
+            // we should not try get credentials with this cred provider.
+            string feedEndPointsJsonEnvVar = Environment.GetEnvironmentVariable(EnvUtil.BuildTaskExternalEndpoints);
+            string uriPrefixesStringEnvVar = Environment.GetEnvironmentVariable(EnvUtil.BuildTaskUriPrefixes);
+            string accessTokenEnvVar = Environment.GetEnvironmentVariable(EnvUtil.BuildTaskAccessToken);
+
+            if (string.IsNullOrWhiteSpace(feedEndPointsJsonEnvVar) == false || string.IsNullOrWhiteSpace(uriPrefixesStringEnvVar) == false || string.IsNullOrWhiteSpace(accessTokenEnvVar) == false)
+            {
+                Verbose(Resources.BuildTaskCredProviderIsUsedError);
+                return false;
+            }
+
             var validHosts = EnvUtil.GetHostsFromEnvironment(Logger, EnvUtil.SupportedHostsEnvVar, new[]
             {
                 ".pkgs.vsts.me", // DevFabric

CredentialProvider.Microsoft/RequestHandlers/RequestHandlerBase.cs

@@ -68,6 +68,8 @@ namespace NuGetCredentialProvider.RequestHandlers
                     var cancelMessage = MessageUtilities.Create(message.RequestId, MessageType.Cancel, message.Method);
                     await connection.SendAsync(cancelMessage, CancellationToken.None);
 
+                    Logger.Verbose(ex.ToString());
+
                     // We must guarantee that exactly one terminating message is sent, so do not fall through to send
                     // the normal response, but also do not rethrow.
                     return;
@@ -89,7 +91,7 @@ namespace NuGetCredentialProvider.RequestHandlers
                 }
 
                 Logger.Verbose(string.Format(Resources.ResponseHandlerException, message.Method, message.RequestId));
-                Logger.Error(ex.ToString());
+                Logger.Verbose(ex.ToString());
                 return false;
             }