[Bug] Format Exception with custom template

**Feature Request or Bug**
Bug

**Describe the feature request or bug**
I created a custom listener profile and receive stack traces when using HTTP grunts (see screenshots).
Basically, there seems to be a string format issue in the HTTP handler:
~~~
fail: Microsoft.AspNetCore.Server.Kestrel[13]
Connection id "0HM7OU2GJC1EL", Request id "0HM7OU2GJC1EL:00000002": An unhandled exception was thrown by the application.
System.FormatException: Input string was not in a correct format.
at System.Text.StringBuilder.FormatError()
at System.Text.StringBuilder.AppendFormatHelper(IFormatProvider provider, String format, ParamsArray args)
at System.String.FormatHelper(IFormatProvider provider, String format, ParamsArray args)
at System.String.Format(String format, Object arg0, Object arg1)
at Covenant.Controllers.HttpListenerController.Route() in /opt/Covenant/Covenant/Controllers/ListenerControllers/HttpListenerController.cs:line 77
at lambda_method(Closure , Object )
at Microsoft.Extensions.Internal.ObjectMethodExecutorAwaitable.Awaiter.GetResult()
at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
at Microsoft.AspNetCore.Routing.EndpointMiddleware.g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication`1 application)
~~~

This had no negative impact for the operation of the grunt, but the uncomfortable feeling remains...

**To Reproduce**
Steps to reproduce the behavior:
Honestly I have no clue what my template does differently than the builtin templates - it is just a slight modification of builtin templates (with an nginx default page instead of Hello World and different parameter names).

**Expected behavior**
No crashes / stack traces.

**Screenshots**

If applicable, add screenshots to help explain your problem.

**Covenant Server Information:**
- OS: Kali
- Docker or Native: native
- Note: I'm using the zeropoint security fork that is used for CRTO. Nevertheless, the bug also exists upstream.

**Browser Information:**
- Browser Edge Chromium
- Version 89

**Target Information (System that implant is running on):**
- OS: Windows 10

**Additional context**

The issue seems to be [here](https://github.com/cobbr/Covenant/blob/master/Covenant/Controllers/ListenerControllers/HttpListenerController.cs#L71) - the replacements performed in the main body of the function does an expansion `{` -> `{{` before replacing the values. This behavior was introduced in commit 15b1fad37f29affb75c4652566d0123c20d5afd9 but it seems it was overlooked in the exception handlers. I adapted the code in the exception handlers in my local build and the errors are gone.