Improve error message returned by LoadAscii (and maybe other Load...)

[mantid-builder]

Original Reporter: @FedeMPouzols

This ticket is very similar to http://trac.mantidproject.org/mantid/ticket/9248 (which was about LoadNexusMonitors).

If you try to load for example UsageData/ls5637.mat you'll get a cryptic log message:

Error in execution of algorithm LoadAscii:
All arguments to init must be positive and non-zero

This can be very misleading to users. It would be very simple to provide a more informative error message to avoid confusion. Check also what happens with other Load... algorithms.

[mantid-builder]

This issue was originally trac ticket 10722