Revise wording of 'initial state' in WFC validation messages

[pefribeiro]

Currently the validator will state that 'A [composite state|state machine] should have exactly one initial state', see instances below:

robochart-textual/circus.robocalc.robochart.textual/src/circus/robocalc/robochart/textual/validation/RoboChartValidator.xtend

Line 378 in b63f57a

'A composite state should have exactly one initial state',

robochart-textual/circus.robocalc.robochart.textual/src/circus/robocalc/robochart/textual/validation/RoboChartValidator.xtend

Line 388 in b63f57a

'A state machine should have exactly one initial state',

robochart-textual/circus.robocalc.robochart.textual/src/circus/robocalc/robochart/textual/validation/RoboChartValidator.xtend

Line 400 in b63f57a

'A state machine or composite state should have exactly one initial state',

Strictly speaking these are 'initial junctions', so would be nice to update the error message to be more accurate.