Fix domain deleting when fail on not exist stack

When stack not exists `findApiId()` fails on `getStack()` and then on `getNestedStack()`. Try block in `removeBasePathMappings()` do not set `externalBasePathExists` flag. As result domain that has other mappings are removed. This fix adds extra check to catch block of `removeBasePathMappings()` if option `preserveExternalPathMappings` is set in stack config (by setting this option user identify that domain should have another mappings) and set the flag `externalBasePathExists` that prevent removing domain.

src/index.ts

@@ -408,6 +408,9 @@ class ServerlessCustomDomain {
             `Unable to remove base path mappings for '${domain.givenDomainName}':\n${err.message}`
           );
         }
+        if (domain.preserveExternalPathMappings) {
+          externalBasePathExists = true;
+        }
       }
 
       if (domain.autoDomain === true && !externalBasePathExists) {