cmake: show an error message when project data is invalid

ChooseCMakeInterfaceJob::fileImportDone() calls tryCMakeServer() as a
fallback when project data is invalid right after reconfiguring.
tryCMakeServer() creates a CMakeServer object and connects to its
signals. These signals are emitted asynchronously, because they are
scheduled after QProcess::started and QProcess::finished signals. But
right after these connections are created, the following events occur
synchronously: CMake::FileApi::ImportJob emits result(),
ChooseCMakeInterfaceJob's base class ExecuteCompositeJob emits result()
and ChooseCMakeInterfaceJob's indirect base class KJob calls
deleteLater(). Thus ChooseCMakeInterfaceJob and its server data member
are destroyed, the fallback is canceled and the user is not notified
about this project configuration error.

The tryCMakeServer() fallback never worked correctly, because CMake
server code is practically unchanged since
76ddc001, which introduced it. I think
no one has ever tested this fallback as its activation is unlikely. I
managed to trigger the fallback by removing read and write filesystem
permissions for the build/.cmake/api/v1/reply directory. As CMake server
mode has been removed since CMake 3.20 in favor of the file-based API,
and KDevelop's file API support works correctly with all CMake versions
where both the file-based API and the server mode are available, there
is no need to make the fallback work correctly. Show an error message
instead.

Setting error and errorText on the ChooseCMakeInterfaceJob object itself
might seem a simpler solution than adding the emitInvalidData option to
CMake::FileApi::ImportJob. However this alternative does not work,
because ExecuteCompositeJob::slotResult() overwrites the values of these
properties with the corresponding values from the last subjob.