update the lint subrequest call to error

[daghack]

Update the lint subrequest call to error when a build error was encountered during linting.

When using the --call=check flag, buildx currently returns 0, even when there was a build error. This correctly updates that behavior and causes the build error to print its source, bringing the behavior in-line with --call=build.

Example

$ BUILDX_EXPERIMENTAL=1 docker buildx --builder=dev build -t trash --call=check . && echo "apple"
[+] Building 0.2s (2/2) FINISHED                                                                                                         docker-container:dev
 => [internal] connecting to local controller                                                                                                            0.0s
 => [internal] load build definition from Dockerfile                                                                                                     0.0s
 => => transferring dockerfile: 171B                                                                                                                     0.0s
FromAsCasing - https://docs.docker.com/go/dockerfile/rule/from-as-casing/
The 'as' keyword should match the case of the 'from' keyword
Dockerfile:7
--------------------
   5 |     COPY $app .
   6 |
   7 | >>> FROM scratch asgard third
   8 |
--------------------

ERROR: dockerfile parse error on line 7: FROM requires either one or three arguments
Dockerfile:7
--------------------
   5 |     COPY $app .
   6 |
   7 | >>> FROM scratch asgard third
   8 |
--------------------

[daghack]

@tonistiigi Can I get a merge here as well?