Improve error message returned by LoadNexusMonitors

[Anders-Markvardsen]

At present if LoadNexusMonitors is used to load a ISIS histogram nexus file, as e.g. AutoTestData/LOQ48127.nxs the error message is:

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

If trying to load a ISIS .raw file, e.g \isis\inst$\NDXHRPD\Instrument\data\cycle_13_5\HRP58608.raw, the error message is:

Error in execution of algorithm LoadNexusMonitors:
NXopen(//isis/inst$/NDXHRPD/Instrument/data/cycle_13_5/HRP58608.raw, 1) failed

The algorithm currently only support loading of SNS or ISIS event raw Nexus files. Hence there could be a check to see if this is satisfied if not return a message along the lines of "blah blah file is not readable by LoadNexusMonitors"

[Anders-Markvardsen]

This issue was originally trac ticket 9248