[WebUI] Impossible to start non-ref scenes job in case any reference scene fails.
When the reference job finishes with errors (e.g., 1 out of 100 scenes fails during homogenization), the WebUI denies to start the jobs with the non-ref scenes. This may make it impossible to run homogenization, e.g. for Landsat, with target sensor Sentinel-2.
Proposed solution: Raise a warning in case the ref-job finishes with errors and start the non-ref-jobs.