Enable better error when invoice cannot be created on crowdfund (#575)

Closes #572

BTCPayServer/Controllers/AppsPublicController.cs

@@ -176,6 +176,8 @@ namespace BTCPayServer.Controllers
             }
             
             store.AdditionalClaims.Add(new Claim(Policies.CanCreateInvoice.Key, store.Id));
+            try
+            {
                 var invoice = await _InvoiceController.CreateInvoiceCore(new Invoice()
                 {
                     OrderId = $"{CrowdfundHubStreamer.CrowdfundInvoiceOrderIdPrefix}{appId}",
@@ -201,6 +203,12 @@ namespace BTCPayServer.Controllers
                     return Ok(invoice.Data.Id);
                 }
             }
+            catch (BitpayHttpException e)
+            {
+                return BadRequest(e.Message);
+            }
+            
+        }
         
 
         [HttpPost]