Fix imex race condition, (#2255)

fix #2254: if the DB was closed without calling stop_io() and then an interrupt arrives (e.g. incoming message), the db was corrupted.

* Add result.log() for logging with less boilerplate code

* Bugfix: Resultify housekeeping() to make it abort if the db is closed instead of just deleting everything

* Require the UI to call dc_stop_io() before backup export

* Prepare a bit better for closed-db: Resultify get_uidvalidity and get_uid_next and let job::load_next() wait until the db is open

About the bug (before this PR):
if the DB was closed without calling stop_io() and then an interrupt arrives (e.g. incoming message):
- I don't know if it downloads the message, but of course at some point the process of receiving the message will fail
- In my test, DC is just in the process of moving a message when the imex starts, but then can't delete the job or update the msg server_uid
- Then, when job::load_next() is called, no job can be loaded. That's why it calls `load_housekeeping_job()`. As `load_housekeeping_job()` can't load the time of the last housekeeping, it assumes we never ran housekeeping and returns a new Housekeeping job, which is immediately executed.
- housekeeping can't find any blobs referenced in the db and therefore deletes almost all blobs.

src/log.rs

@@ -1,4 +1,6 @@
-//! # Logging macros
+//! # Logging
+
+use crate::context::Context;
 
 #[macro_export]
 macro_rules! info {
@@ -58,3 +60,39 @@ macro_rules! emit_event {
         $ctx.emit_event($event);
     };
 }
+
+pub trait LogExt<T> {
+    /// Emits a warning if the receiver contains an Err value.
+    ///
+    /// Returns an [`Option<T>`] with the `Ok(_)` value, if any:
+    /// - You won't get any warnings about unused results but can still use the value if you need it
+    /// - This prevents the same warning from being printed to the log multiple times
+    ///
+    /// Thanks to the [track_caller](https://blog.rust-lang.org/2020/08/27/Rust-1.46.0.html#track_caller)
+    /// feature, the location of the caller is printed to the log, just like with the warn!() macro.
+    #[track_caller]
+    fn log(self, context: &Context) -> Option<T>;
+}
+
+impl<T> LogExt<T> for anyhow::Result<T> {
+    #[track_caller]
+    fn log(self, context: &Context) -> Option<T> {
+        match self {
+            Err(e) => {
+                let location = std::panic::Location::caller();
+                // We are using Anyhow's .context() and to show the inner error, too, we need the {:#}:
+                let full = format!(
+                    "{file}:{line}: {e:#}",
+                    file = location.file(),
+                    line = location.line(),
+                    e = e
+                );
+                // We can't use the warn!() macro here as the file!() and line!() macros
+                // don't work well with #[track_caller]
+                emit_event!(context, crate::EventType::Warning(full));
+                None
+            }
+            Ok(v) => Some(v),
+        }
+    }
+}