Shorter short error messages.

mediapreview/exceptions.py

@@ -154,7 +154,15 @@ def onlyoffice_no_fileurl_error(snippet: str | None = None) -> OnlyOfficeError:
 
 
 def backend_error(backend: str, message: str) -> PreviewBackendError:
-    short = message.splitlines()[0][:60]
+    short = message.splitlines()[0]
+    # Many backend messages look like "source: summary: detail ...".
+    # Drop the source prefix and any trailing detail so the short label
+    # is usable in UIs with limited space.
+    if ": " in short:
+        short = short.split(": ", 1)[1]
+    if ": " in short:
+        short = short.split(": ", 1)[0]
+    short = short[:60]
     return PreviewBackendError(
         f"[{backend}] preview failed: {message}",
         short,

tests/test_office.py

@@ -72,6 +72,15 @@ def test_backend_error_pipeline_backend():
     assert isinstance(err, PreviewBackendError)
 
 
+def test_backend_error_short_message_strips_source_and_detail():
+    """Backend messages like "source: summary: detail" become just the summary."""
+    err = backend_error(
+        "vips",
+        "pyvips: cannot decode image: unable to load from file b'/mnt/c/Users...",
+    )
+    assert err.short == "cannot decode image"
+
+
 def test_error_pickle_round_trip():
     """Exceptions survive pickling (the worker pool wire) intact."""
     err = onlyoffice_error_from_code("-8")