Lwtreslib: provide exception-catching functions

Context

We have multiple ways to deal with errors: result, tzresult, option, exception. Occasionally, we write code that bridges the gap between those different ways. (E.g., when we call a library that uses exceptions inside a function that uses results.)

Typical example: !2765 (comment 555727601)

This MR provides a few helper functions to smooth the transition between the different kinds of error management.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.