[wasm] IcallTableGenerator: For unsupported types, log a warning instead

.. of throwing an exception.

```
 The "IcallTableGenerator" task failed unexpectedly. [C:\uselessStuff\GameBuddy\src\Client\GameBuddy.Client.csproj]
 System.NotImplementedException: System.Enum [C:\uselessStuff\GameBuddy\src\Client\GameBuddy.Client.csproj]
    at IcallTableGenerator.AppendType(StringBuilder sb, Type t) [C:\uselessStuff\GameBuddy\src\Client\GameBuddy.Client.csproj]
    at IcallTableGenerator.ProcessType(Type type) [C:\uselessStuff\GameBuddy\src\Client\GameBuddy.Client.csproj]
    at IcallTableGenerator.GenIcallTable(String runtimeIcallTableFile, String[] assemblies) [C:\uselessStuff\GameBuddy\src\Client\GameBuddy.Client.csproj]
    at IcallTableGenerator.Execute() [C:\uselessStuff\GameBuddy\src\Client\GameBuddy.Client.csproj]
    at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute() [C:\uselessStuff\GameBuddy\src\Client\GameBuddy.Client.csproj]
    at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask) [C:\uselessStuff\GameBuddy\src\Client\GameBuddy.Client.csproj]
```

becomes:

```
C:\Program Files\dotnet\packs\Microsoft.NET.Runtime.WebAssembly.Sdk\7.0.0-alpha.1.21551.1\Sdk\WasmApp.Native.targets(258,5):
    warning : Failed to generate icall function for [System.Private.CoreLib]System.Enum::InternalHasFlag because type System.Enum is not supported for parameter named flags. Ignoring. [C:\Users\Ankit Jain\bl0\bl0.csproj]
```

Prompted by dotnet#61053

src/tasks/WasmAppBuilder/IcallTableGenerator.cs

@@ -31,8 +31,16 @@ public class IcallTableGenerator : Task
 
     public override bool Execute()
     {
-        GenIcallTable(RuntimeIcallTableFile!, Assemblies!.Select(item => item.ItemSpec).ToArray());
-        return true;
+        try
+        {
+            GenIcallTable(RuntimeIcallTableFile!, Assemblies!.Select(item => item.ItemSpec).ToArray());
+            return !Log.HasLoggedErrors;
+        }
+        catch (LogAsErrorException laee)
+        {
+            Log.LogError(laee.Message);
+            return false;
+        }
     }
 
     //
@@ -152,20 +160,9 @@ private void ProcessType (Type type)
             icallClass.Icalls.TryGetValue (method.Name, out icall);
             if (icall == null)
             {
-                // Then with signature
-                var sig = new StringBuilder (method.Name + "(");
-                int pindex = 0;
-                foreach (var par in method.GetParameters())
-                {
-                    if (pindex > 0)
-                        sig.Append (',');
-                    var t = par.ParameterType;
-                    AppendType (sig, t);
-                    pindex++;
-                }
-                sig.Append (')');
-                if (icallClass.Icalls.ContainsKey (sig.ToString ()))
-                    icall = icallClass.Icalls [sig.ToString ()];
+                string? methodSig = BuildSignature(method, className);
+                if (methodSig != null && icallClass.Icalls.ContainsKey (methodSig))
+                    icall = icallClass.Icalls [methodSig];
             }
             if (icall == null)
                 // Registered at runtime
@@ -178,6 +175,33 @@ private void ProcessType (Type type)
 
         foreach (var nestedType in type.GetNestedTypes())
             ProcessType(nestedType);
+
+        string? BuildSignature(MethodInfo method, string className)
+        {
+            // Then with signature
+            var sig = new StringBuilder (method.Name + "(");
+            int pindex = 0;
+            foreach (var par in method.GetParameters())
+            {
+                if (pindex > 0)
+                    sig.Append (',');
+                var t = par.ParameterType;
+                try
+                {
+                    AppendType (sig, t);
+                }
+                catch (NotImplementedException nie)
+                {
+                    Log.LogWarning($"Failed to generate icall function for '[{method.DeclaringType!.Assembly.GetName().Name}] {className}::{method.Name}'" +
+                                    $" because type '{nie.Message}' is not supported for parameter named '{par.Name}'. Ignoring.");
+                    return null;
+                }
+                pindex++;
+            }
+            sig.Append (')');
+
+            return sig.ToString();
+        }
     }
 
     // Append the type name used by the runtime icall tables