Fix error messages from backends

[presenthee]

This PR handles #73.

Main Changes:

• Added Error module and put Source.error functions to util/error.ml.
• Cleaned up failure related to values into functions in al_util.
• Avoided overusing failwith in backends. Use Error.error if possible.
• Changed error messages to be more useful by adding detailed explanations.

Changes specific to the backend-interpreter:
For AL-level debugging, new exceptions were added.

• Backend_interpreter.Exception.Error is raised when there is an exception while stepping an instruction. (by try_step_wasm, try_step_instr)
• Backend_interpreter.Exception.Error contains step(instruction), line and al function name information. These are printed with the original exception message.
• Added other exceptions for indicating unexpected error related to calling functions or construction of value.

[rossberg]

This looks good. I added a few comments where the message possibly is rather opaque for end users.

[rossberg]

Thanks!