Mark invalid external repo specifiers as configuration error

This is a workaround — the proper fix is to better separate out user input vs the Action in `database init` so we can better attribute configuration errors.

src/cli-errors.ts

@@ -124,6 +124,7 @@ export enum CliConfigErrorCategory {
   IncompatibleWithActionVersion = "IncompatibleWithActionVersion",
   InitCalledTwice = "InitCalledTwice",
   InvalidConfigFile = "InvalidConfigFile",
+  InvalidExternalRepoSpecifier = "InvalidExternalRepoSpecifier",
   InvalidSourceRoot = "InvalidSourceRoot",
   MavenBuildFailed = "MavenBuildFailed",
   NoBuildCommandAutodetected = "NoBuildCommandAutodetected",
@@ -188,6 +189,11 @@ export const cliErrorsConfig: Record<
       new RegExp("The supplied config file is empty"),
     ],
   },
+  [CliConfigErrorCategory.InvalidExternalRepoSpecifier]: {
+    cliErrorMessageCandidates: [
+      new RegExp("Specifier for external repository is invalid"),
+    ],
+  },
   // Expected source location for database creation does not exist
   [CliConfigErrorCategory.InvalidSourceRoot]: {
     cliErrorMessageCandidates: [new RegExp("Invalid source root")],