pisearch.go: make it compile in go 1.0.3

Before 1.1, you had to explicitly end functions with a `return`
statement. http://golang.org/doc/go1.1#return

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
pisearch.go: print full filename when can't find it

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
README.md: clarify where pi files go

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>