lib/gpg-verify: Add an OstreeGpgError error domain

Add a new error domain for GPG signing/verification errors, and use it
throughout libostree for describing verification errors. This replaces
various uses of G_IO_ERROR_FAILED, and one instance of
G_IO_ERROR_NOT_FOUND (for which some code in ot-builtin-show.c had to be
changed to ensure it was still handled correctly).

The use of a separate error domain allows failures in GPG operations to
be handled separately from network failures (where the summary file
could not be found to be downloaded, for example) or timeouts.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1064

Closes: #1071
Approved by: mbarnes

src/libostree/ostree-gpg-verify-result.h

@@ -137,4 +137,25 @@ _OSTREE_PUBLIC
 gboolean ostree_gpg_verify_result_require_valid_signature (OstreeGpgVerifyResult *result,
                                                            GError **error);
 
+/**
+ * OstreeGpgError:
+ * @OSTREE_GPG_ERROR_NO_SIGNATURE: A signature was expected, but not found.
+ * @OSTREE_GPG_ERROR_INVALID_SIGNATURE: A signature was malformed.
+ * @OSTREE_GPG_ERROR_MISSING_KEY: A signature was found, but was created with a key not in the configured keyrings.
+ *
+ * Errors returned by signature creation and verification operations in OSTree.
+ * These may be returned by any API which creates or verifies signatures.
+ *
+ * Since: 2017.10
+ */
+typedef enum {
+  OSTREE_GPG_ERROR_NO_SIGNATURE = 0,
+  OSTREE_GPG_ERROR_INVALID_SIGNATURE,
+  OSTREE_GPG_ERROR_MISSING_KEY,
+} OstreeGpgError;
+
+_OSTREE_PUBLIC
+GQuark ostree_gpg_error_quark (void);
+#define OSTREE_GPG_ERROR (ostree_gpg_error_quark ())
+
 G_END_DECLS