Provide additional information during errors

This ought to help developers diagnose where the error occurred. The current stack trace provides no actionable information and requires going through the whole Interface searching for violations.

```
Unhandled Exception: System.ArgumentException: URL has parameter member, but no method parameter matches
   at Refit.RestMethodInfo.BuildParameterMap(String relativePath, List`1 parameterInfo) in D:\a\1\s\Refit\RestMethodInfo.cs:line 143
   at Refit.RestMethodInfo..ctor(Type targetInterface, MethodInfo methodInfo, RefitSettings refitSettings) in D:\a\1\s\Refit\RestMethodInfo.cs:line 64
   at Refit.RequestBuilderImplementation`1..ctor(RefitSettings refitSettings) in D:\a\1\s\Refit\RequestBuilderImplementation.cs:line 58
   at Refit.RequestBuilderFactory.Create[T](RefitSettings settings) in D:\a\1\s\Refit\RequestBuilderFactory.cs:line 17
   at Refit.RequestBuilder.ForType[T](RefitSettings settings) in D:\a\1\s\Refit\RequestBuilder.cs:line 24
   at Refit.RestService.For[T](HttpClient client, RefitSettings settings) in D:\a\1\s\Refit\RestService.cs:line 28
   at Refit.RestService.For[T](HttpClient client) in D:\a\1\s\Refit\RestService.cs:line 33
   at MyCode.Program.Main(String[] args) in /Users/me/Documents/MyCode/MyCode/Program.cs:line 16
```

Refit/RestMethodInfo.cs

@@ -94,7 +94,7 @@ public RestMethodInfo(Type targetInterface, MethodInfo methodInfo, RefitSettings
 
             var ctParams = methodInfo.GetParameters().Where(p => p.ParameterType == typeof(CancellationToken)).ToList();
             if(ctParams.Count > 1) {
-                throw new ArgumentException("Argument list can only contain a single CancellationToken");
+                throw new ArgumentException($"Argument list to method \"{methodInfo.Name}\" can only contain a single CancellationToken");
             }
 
             CancellationToken = ctParams.FirstOrDefault();
@@ -117,7 +117,7 @@ void VerifyUrlPathIsSane(string relativePath)
             return;
 
         bogusPath:
-            throw new ArgumentException("URL path must be of the form '/foo/bar/baz'");
+            throw new ArgumentException($"URL path {relativePath} must be of the form '/foo/bar/baz'");
         }
 
         Dictionary<int, string> BuildParameterMap(string relativePath, List<ParameterInfo> parameterInfo)
@@ -137,7 +137,7 @@ Dictionary<int, string> BuildParameterMap(string relativePath, List<ParameterInf
             foreach (var match in parameterizedParts) {
                 var name = match.Groups[1].Value.ToLowerInvariant();
                 if (!paramValidationDict.ContainsKey(name)) {
-                    throw new ArgumentException(string.Format("URL has parameter {0}, but no method parameter matches", name));
+                    throw new ArgumentException($"URL {relativePath} has parameter {name}, but no method parameter matches");
                 }
 
                 ret.Add(parameterInfo.IndexOf(paramValidationDict[name]), name);
@@ -290,7 +290,7 @@ void DetermineReturnTypeInfo(MethodInfo methodInfo)
             return;
 
         bogusMethod:
-            throw new ArgumentException("All REST Methods must return either Task<T> or IObservable<T>");
+            throw new ArgumentException($"Method \"{methodInfo.Name}\" is invalid. All REST Methods must return either Task<T> or IObservable<T>");
         }     
     }
 }