main: prefer .context to map_err(|e| anyhow!(...))
main: prefer .context to map_err(|e| anyhow!(...))

`anyhow::context` is more idiomatic way to handle errors than mapping an
error by `map_err(|e| anyhow!(...))`. However, main.rs uses the latter
right now.

This commit rewrites such error handling to the `.context` manner.

Note that this commit does not rewrite all `map_err` because some of
them requires refactoring various functions which return the unit type
as their error like `Result<..., ()>`. Because the unit type does not
implement `std::Error`, we cannot wrap it with `.context`.

BUG=None
TEST=build succeeds

Change-Id: I5829db1ed98bafd3d61a165172a009ab8db29d53
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3831834
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Reviewed-by: David Stevens <2f87ba95ff5d327328bd68c2e86a4d9f2ba99bd0@chromium.org>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Takaya Saeki <b41986321ffdf64f9ab5b24fec89eb23749dcec8@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm