Fix #181: Fix crash for unsupported field types

This change adds proper error handling for input prompt requests for field
types that we don't yet support like SecureString and PSCredential.
Previously any Read-Host or Get-Credential call which prompts for either
of these types would crash the language and debugging services.  The fix
causes errors to be written to the host stating that these commands and
field types are not yet supported.

Author: daviwil

src/PowerShellEditorServices/Console/FieldDetails.cs

@@ -9,6 +9,7 @@
 using System.Collections.Generic;
 using System.Management.Automation;
 using System.Management.Automation.Host;
+using System.Security;
 
 namespace Microsoft.PowerShell.EditorServices.Console
 {
@@ -89,7 +90,17 @@ public FieldDetails(
             this.IsMandatory = isMandatory;
             this.DefaultValue = defaultValue;
 
-            if (typeof(IList).IsAssignableFrom(fieldType))
+            if (typeof(SecureString) == fieldType)
+            {
+                throw new NotSupportedException(
+                    "Input fields of type 'SecureString' are currently not supported.");
+            }
+            else if (typeof(PSCredential) == fieldType)
+            {
+                throw new NotSupportedException(
+                    "Input fields of type 'PSCredential' are currently not supported.");
+            }
+            else if (typeof(IList).IsAssignableFrom(fieldType))
             {
                 this.IsCollection = true;
                 this.ElementType = typeof(object);

src/PowerShellEditorServices/Session/SessionPSHostUserInterface.cs

@@ -154,7 +154,8 @@ public override PSCredential PromptForCredential(
             PSCredentialTypes allowedCredentialTypes, 
             PSCredentialUIOptions options)
         {
-            throw new NotImplementedException();
+            throw new NotSupportedException(
+                "'Get-Credential' is not yet supported.");
         }
 
         public override PSCredential PromptForCredential(
@@ -163,7 +164,13 @@ public override PSCredential PromptForCredential(
             string userName, 
             string targetName)
         {
-            throw new NotImplementedException();
+            return this.PromptForCredential(
+                caption,
+                message,
+                userName,
+                targetName,
+                PSCredentialTypes.Default,
+                PSCredentialUIOptions.Default);
         }
 
         public override PSHostRawUserInterface RawUI
@@ -198,7 +205,8 @@ public override string ReadLine()
 
         public override SecureString ReadLineAsSecureString()
         {
-            throw new NotImplementedException();
+            throw new NotSupportedException(
+                "'Read-Host -AsSecureString' is not yet supported.");
         }
 
         public override void Write(