aconfig: improve error messages

Propagate anyhow errors to main.rs and commands.rs to improve the error
message. As an example. instead of just "bad flag declaration: exactly
one bug required", aconfig will now print the following.

---- 8< ----
Error: failed to create cache

Caused by:
    0: failed to parse build/make/tools/aconfig/tests/test.aconfig
    1: bad flag declaration: missing description
---- >8 ----

Error messages can be improved further by including additional
information in the protos.rs error cases. This will be handled in a
follow-up CL.

Bug: 290300657
Test: manual: introduce error in an aconfig file and run `m all_aconfig_declarations`
Test: atest aconfig.test
Change-Id: Id278f4877e5794b95913ae8ba0ca3ee211293f38