Incremental fast error backoff

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

@@ -4076,8 +4076,8 @@ class Core {
             return networkState;
         }
 
-        void setBackoff(int value) {
-            this.backoff = value;
+        void setBackoff(int seconds) {
+            this.backoff = seconds;
         }
 
         int getBackoff() {

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

@@ -1555,8 +1555,13 @@ public class ServiceSynchronize extends ServiceBase implements SharedPreferences
                         EntityLog.log(ServiceSynchronize.this,
                                 account.name + " fast errors=" + errors +
                                         " last connected: " + new Date(account.last_connected));
-                        if (errors >= FAST_ERROR_COUNT)
-                            state.setBackoff(CONNECT_BACKOFF_AlARM_START * 60);
+                        if (errors >= FAST_ERROR_COUNT) {
+                            int scale = errors - FAST_ERROR_COUNT + 1;
+                            int backoff = Math.max(CONNECT_BACKOFF_AlARM_START * scale, CONNECT_BACKOFF_AlARM_MAX);
+                            EntityLog.log(ServiceSynchronize.this,
+                                    account.name + " fast error backoff=" + backoff);
+                            state.setBackoff(backoff * 60);
+                        }
                     }
 
                     // Report account connection error