Add message when --check does not produce warnings or errors

[daghack]

Currently, in the case that --check produces no errors or warnings, there is no output to clarify this. This adds a simple user facing message in the case to indicate such when it occurs.

~/example$ BUILDX_EXPERIMENTAL=1 docker --debug buildx --builder=dev build --check .
[+] Building 0.1s (3/3) FINISHED                                                                                                         docker-container:dev
 => [internal] connecting to local controller                                                                                                            0.0s
 => [internal] load build definition from Dockerfile                                                                                                     0.0s
 => => transferring dockerfile: 105B                                                                                                                     0.0s
 => [internal] load .dockerignore                                                                                                                        0.0s
 => => transferring context: 2B                                                                                                                          0.0s

Becomes

~/example$ BUILDX_EXPERIMENTAL=1 docker --debug buildx --builder=dev build --check .
[+] Building 0.2s (3/3) FINISHED                                                                                                         docker-container:dev
=> [internal] connecting to local controller                                                                                                            0.0s
=> [internal] load build definition from Dockerfile                                                                                                     0.0s
=> => transferring dockerfile: 105B                                                                                                                     0.0s
=> [internal] load .dockerignore                                                                                                                        0.0s
=> => transferring context: 2B                                                                                                                          0.0s
Check complete, no warnings found.

[tonistiigi]

This should not be printed if format is json. Same for the newline in the Error formatter.