users guide: Add documentation for custom compile-time errors
users guide: Add documentation for custom compile-time errors

(cherry picked from commit 568736d757d3e0883b0250e0b948aeed646c20b5)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc