Document sign key handling

FAQ.md

@@ -449,6 +449,10 @@ First of all you need to install and configure [OpenKeychain](https://f-droid.or
 To encrypt and send a message just check the menu *Encrypt* and the message will be encrypted on sending.
 Similarly, to decrypt a received message, just select the menu *Decrypt* in the expanded message view.
 
+The first time you send an encrypted message you might be asked for a sign key.
+FairEmail will automatically store the sign key ID in the selected identity for the next time.
+If you need to reset the sign key, just save the identity to clear the sign key ID again.
+
 You can enable *Encrypt by default* in the identity settings, which replaces *Send* by *Encrypt and send*.
 
 FairEmail will send the [Autocrypt](https://autocrypt.org/) headers for other email clients.

app/src/main/java/eu/faircode/email/DaoIdentity.java

@@ -85,7 +85,7 @@ public interface DaoIdentity {
     int setIdentityConnected(long id, long last_connected);
 
     @Query("UPDATE identity SET sign_key = :sign_key WHERE id = :id")
-    int setIdentitySignKey(long id, long sign_key);
+    int setIdentitySignKey(long id, Long sign_key);
 
     @Query("UPDATE identity SET error = :error WHERE id = :id")
     int setIdentityError(long id, String error);

app/src/main/java/eu/faircode/email/FragmentCompose.java

@@ -1486,11 +1486,13 @@ public class FragmentCompose extends FragmentBase {
                         return result.getParcelableExtra(OpenPgpApi.RESULT_INTENT);
 
                     case OpenPgpApi.RESULT_CODE_ERROR:
+                        if (identity != null)
+                            db.identity().setIdentitySignKey(identity.id, null);
                         OpenPgpError error = result.getParcelableExtra(OpenPgpApi.RESULT_ERROR);
                         if (error == null)
                             throw new IllegalArgumentException("Unknown error");
                         else
-                            throw new IllegalArgumentException(error.getMessage());
+                            throw new IllegalArgumentException(error.getMessage() + " (" + error.getErrorId() + ")");
 
                     default:
                         throw new IllegalArgumentException("Unknown result code=" + resultCode);