Refs #8855. Check first good bin value on every access.

Reset it to default if empty/invalid value is specified and output the
appropriate warning.
Refs #8855. Refactor plotFromTime and plotToTime.

ploFromTime now uses firstGoodData() to avoid checking/resetting the
value in two plaves. Both plotFromTime and plotToTime now behave in the
same way firstGoodData() does.
Refs #8855. Catch exceptions from AlphaCalc.
Merge remote-tracking branch 'origin/bugfix/8855_muon_guess_alpha_exception'

Conflicts:
	Code/Mantid/MantidQt/CustomInterfaces/src/MuonAnalysis.cpp
Re #8855