better error message when importing (into a variable)

Students were flumoxed when they couldn't import into a variable from a .csv file on windows (snap silently did nothing).  If they simply changed the extension to .txt things worked. I looked at OKaying the .csv type in snap, but (on windows) the file was "application/ms ..." which was ugly and maybe not distinguishable from a regular excel file!  

This just adds an error message. 

Additionally, I had it read only the last file if multiple were picked, since reading multiple files seemed a waste and made error reporting trickier.  (On windows you can't pick more than one). 

This fix removes a "feature", though, where Snap would do something useful if both text files and non-text files were picked -- this fix only ever looks at the last file in the pick list.