Reporting stage

• The Prusti client reports compilation and verification errors.

In this final stage, any errors and warnings that occurred during verification are reported to the user.

• prusti-viper/src/verifier.rs - Verifier::verify - collection of errors from verification call.
• prusti-viper/src/encoder/errors/prusti_error.rs - PrustiError::emit - emission of Prusti errors as messages and source file spans into the compiler environment.