feat: implement web service

Signed-off-by: The one with the braid <info@braid.business>

lib/pkpass/error.dart

@@ -0,0 +1,30 @@
+abstract class PKPassError extends Error {
+  final String message;
+
+  PKPassError({required this.message});
+
+  @override
+  String toString() => 'PKPassError: $message';
+}
+
+class InvalidEncodingError extends PKPassError {
+  InvalidEncodingError() : super(message: 'Input not in ZIP format.');
+}
+
+class ManifestNotFoundError extends PKPassError {
+  ManifestNotFoundError()
+      : super(message: 'No manifest.json found in PKPass archive.');
+}
+
+class ManifestChecksumError extends PKPassError {
+  final String expected;
+  final String actual;
+
+  ManifestChecksumError({
+    required this.expected,
+    required this.actual,
+  }) : super(
+          message:
+              'Manifest sha1 checksum missmatch, expected $expected, actual $actual',
+        );
+}