Factor Error and Expected boilerplate return-macros

[mtrofin]

Examples: RETURN_IF_ERROR and ASSIGN_OR_RETURN or these with a slightly different name.

The idea is to place them in Error.h

[mtrofin]

FWIW, it may be more tricky than it seems. The Error case is straightforward, the Expect<T> isn't. In the OutputSizeLimitTest.cpp example, the goal is to assign Var to *Expect, while in the ResourceScriptParser.cpp example, Var gets assigned Expect. Then, here it gets more interesting, with references and const references.

Maybe we end up with a few macros, but then again, maybe having these macros n-plicated isn't so bad afterall.

Also:

• and there's also ErrorOr.
• ASSIGN_OR_ERROR (and RETURN_IF_ERROR) is a very good name, it's also unfortunately taken by abseil. If these appear in a header, it will complicate codebases where llvm and abseil mix.