Fixed issue #957 : Removed duplicate error and replaced old duplicate when used

This mr aims to close issue #957 (closed).

I removed "SPACECRAFT_MASS_BECOMES_NEGATIVE" and kept "NOT_POSITIVE_SPACECRAFT_MASS" as it was more generic.

Every error that used "SPACECRAFT_MASS_BECOMES_NEGATIVE" are now replaced by "NOT_POSITIVE_SPACECRAFT_MASS".