Consistently raise FileNotFound errors across each load_*() functions.

[ns-rse]

In the recent v0.0.2 release the raising of errors if files do not exist at the given location /can not be found isn't quite consistent across the supported filetypes.

File Extension	Raises FileNotFound error
.spm	✅
.jpk	✅
.ibw	❎
.gwy	✅
asd.py	❎

ibw.py

The load_ibw() function does have an explicit exception check for FileNotFound but within this there is no raise to ensure the error is raised when encountered. Instead it progresses and we observe an UnboundLocalError.

asd.py

The load_asd() function does not currently have an explicit except/raise for FileNotFound.