Decide on the convention for error messages

For example:

`RUNTIME ERROR: couldn't open import "no chance a file with this name exists": no match locally or in the Jsonnet library paths`

vs

`RUNTIME ERROR: Couldn't open import "no chance a file with this name exists": no match locally or in the Jsonnet library paths.`

We need to decide how we want it and keep it consistent. It may require changing APIs slightly, so that errors from Go (file not found, etc.) won't break our convention and that golint is happy.