Return code after execution

In order to have a status of the execution, S1Processor shall return a status code after execution.

In a first version, the return code could be 0 if execution was performed without error and -1 if an error occurred.

In a later version, the code could inform with a more accurately. For example:

• -1 : Downloading error (from EODAG)
• -2 : Download incomplete (data not available online (#71 (closed))
• -3 : OTB pipeline errors
• -4 : output disk full
• -5 : cache disk full (with --cache-before-ortho)