software_center_download: When file is not found, it ignores searching locally and downloads alternative

[marcelmamula]

Issue

When SAP discontinues SWPM version, download correctly identifies that it is not present and goes for alternative.
But this will lead to issue sap-linuxlab/community.sap_install#1031 if previous version is already present, resulting in crashed sap_swpm role because there will be multiple SWPM files present.