fix: show rack elevation errors in the exported document

Rack SVG failures were only logged, leaving racks silently without an
image. The error is now written into the document next to the rack so
the cause (e.g. missing rack_elevation.css in STATIC_ROOT) is visible.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

netbox_customer_export/sections.py

@@ -172,13 +172,17 @@ def racks(scope, opts):
                     )
                 )
                 if opts.get("rack_svg"):
-                    images = []
+                    images, errors = [], []
                     for face in opts.get("rack_faces") or ["front"]:
-                        svg = render_rack_svg(rack, face, scope.user)
+                        caption = "Vorderseite" if face == "front" else "Rückseite"
+                        svg, error = render_rack_svg(rack, face, scope.user)
                         if svg:
-                            images.append((svg, "Vorderseite" if face == "front" else "Rückseite"))
+                            images.append((svg, caption))
+                        else:
+                            errors.append(f"Rack-Ansicht ({caption}) konnte nicht erzeugt werden: {error}")
                     if images:
                         blocks.append(IMAGES(images))
+                    blocks += [P(e) for e in errors]
                 if opts.get("rack_devices"):
                     devs = scope.restrict(rack.devices.all()).select_related("device_type__manufacturer").order_by("-position", "name")
                     if devs.exists():