Never print directly from deserialization code

[ras0219-msft]

SPDX license parsing warnings were being emitted directly to stdout during manifest deserialization. We should never do this.

Drive-by fixes:

1. I've added a missing trailing newline as part of print_error_message().
2. Added detection of error: as another valid "error" message.

New behavior:

$ vcpkg format-manifest vcpkg.json
/workspaces/vcpkg-tool/vcpkg.json: $.license (an SPDX license expression): warning: Unknown license identifier 'hello'. Known values are listed at https://spdx.org/licenses/
    on expression: hello
                   ^
Failed to parse manifest file: /workspaces/vcpkg-tool/vcpkg.json
error: while loading /workspaces/vcpkg-tool/vcpkg.json:
$ (a manifest): missing required field 'name' (an identifier)
$ (a manifest): expected a versioning field (example: version-string)

[BillyONeal]

I like this. Would you willing to add BillyONeal@2975acd ?

[ras0219-msft]

Pushed