OutputExecuteJob: expand the process error warning

When CMake configuration of a project fails, the following warnings
appear in KDeveolp's output:
kdevplatform.outputview: QProcess::UnknownError "Unknown error"
kdevelop.plugins.cmake: couldn't load project successfully "<project name>" 200 "Exit code 1"

Finding the code that prints the kdevplatform.outputview warning is not
trivial. Even after finding the relevant qCWarning line in
OutputExecuteJob::childProcessError(), there is no way to tell which
process exited with the printed error.

Print the "process error:" prefix to facilitate finding the qCWarning
line and the command line of the process to specify where the error
comes from. For example:
kdevplatform.outputview: process error: QProcess::UnknownError "Unknown error" , the command line: "/usr/bin/cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DCMAKE_INSTALL_PREFIX=/path/to/install-prefix -DCMAKE_BUILD_TYPE=Debug /path/to/project"