Make the input parser more informative

[ChasingNeutrons]

First issue of the new repository!

I write a gargantuan input file, only to crash when I run it with the error:
"There is unparsed content. Missing a token ';' or '{...}'"

That's certainly something, but it would be nice to pin down a line, or to spit out some of the input about the point where it crashes out to speed up error hunting. Otherwise, the search may be quite lengthy. I'm sure everyone would appreciate this!

I suppose this would involve editing the input parser and adding a few small functions. Hopefully not too much effort would be required for a big benefit.