elaborate on Unreachable exception

packages/astro/src/i18n/index.ts

@@ -157,7 +157,7 @@ export function getPathByLocale(locale: string, locales: Locales): string {
 			}
 		}
 	}
-	throw new Error("Unreachable");
+	throw new Unreachable();
 }
 
 /**
@@ -172,14 +172,14 @@ export function getLocaleByPath(path: string, locales: Locales): string {
 			if (locale.path === path) {
 				// the first code is the one that user usually wants
 				const code = locale.codes.at(0);
-				if (code === undefined) throw new Error("Unreachable");
+				if (code === undefined) throw new Unreachable();
 				return code;
 			}
 		} else if (locale === path) {
 			return locale;
 		}
 	}
-	throw new Error("Unreachable");
+	throw new Unreachable();
 }
 
 /**
@@ -241,3 +241,14 @@ function peekCodePathToUse(locales: Locales, locale: string): undefined | string
 
 	return undefined;
 }
+
+class Unreachable extends Error {
+	constructor() {
+		super(
+			"Astro encountered an unexpected line of code.\n" +
+			"In most cases, this is not your fault, but a bug in astro code.\n" +
+			"If there isn't one already, please create an issue.\n" +
+			"https://astro.build/issues"
+		);
+	}
+}