Unresolved conflicts: Warn before or deny some data layer actions

[skyper]

There are some data layer actions which are dangerous and may lead to problems.
One example:

What steps will reproduce the problem?

1. have a data layer with unresolved conflicts (e.g. reverting an old changeset containing changes in long route relations)
2. have another data layer with up to date data of the region
3. merge layers without resolving conflicts

What is the expected result?

Actions is denied or at least a big warning is shown about unresolved conflicts

What happens instead?

Action is performed and conflicts are lost

Please provide any additional information below. Attach a screenshot if possible.

In my case, as I know some problems with merge, I always duplicate layers before merging but duplicate does not warn nor does it keep the conflicts.
So I ended up with:

SEVERE: org.openstreetmap.josm.io.OsmApiException: ResponseCode=412, Error Header=<Relation 9708570 requires the nodes with id in 5760376749,6138235465, which either do not exist, or are not visible.>

and

WARNING: JOSM expected to find primitive [node 6138235465] in dataset but it is not there. Please report this at https://josm.openstreetmap.de. This is not a critical error, it should be safe to continue in your work

We have a warning before upload but:

• merge (selection)
• save (as)
• save session
• upload selection
• duplicate (might simply duplicate keeping all conflicts)

all suffer similar problems

Relative:URL: ^/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2019-12-24 11:33:02 +0100 (Tue, 24 Dec 2019)
Revision:15611
Build-Date:2019-12-25 02:31:00
URL:https://josm.openstreetmap.de/svn/trunk

Identification: JOSM/1.5 (15611 en) Linux Debian GNU/Linux 10 (buster)
Memory Usage: 414 MB / 964 MB (130 MB allocated, but free)
Java version: 11.0.5+10-post-Debian-1deb10u1, Debian, OpenJDK 64-Bit Server VM
Dataset consistency test: No problems found

Plugins:
+ reverter (35248)
+ tag2link (35248)
+ undelete (35248)
+ utilsplugin2 (35248)

Last errors/warnings:
- E: javax.net.ssl.SSLException: Read timed out. Cause: java.net.SocketTimeoutException: Read timed out
- E: java.util.concurrent.ExecutionException: org.openstreetmap.josm.io.OsmTransferException: Could not connect to the OSM server. Please check your internet connection.. Cause: org.openstreetmap.josm.io.OsmTransferException: Could not connect to the OSM server. Please check your internet connection.. Cause: javax.net.ssl.SSLException: Read timed out. Cause: java.net.SocketTimeoutException: Read timed out
- E: Error header: Relation 9708570 requires the nodes with id in 5760376749,6138235465, which either do not exist, or are not visible.
- W: Error header "Relation 9708570 requires the nodes with id in 5760376749,6138235465, which either do not exist, or are not visible." did not match with an expected pattern
- E: org.openstreetmap.josm.io.OsmApiException: ResponseCode=412, Error Header=<Relation 9708570 requires the nodes with id in 5760376749,6138235465, which either do not exist, or are not visible.>
- E: Precondition violation - org.openstreetmap.josm.gui.widgets.HtmlPanel[,0,0,0x0,invalid,layout=java.awt.BorderLayout,alignmentX=0.0,alignmentY=0.0,border=,flags=9,maximumSize=,minimumSize=,preferredSize=]
- W: JOSM expected to find primitive [node 5760376749] in dataset but it is not there. Please report this at https://josm.openstreetmap.de. This is not a critical error, it should be safe to continue in your work.
- E: java.lang.Exception

[skyper]

We already have a warning for "save (as)" but no warning for "upload selection" and "save session".