[lldb-dap] Missing feedback if `lldb-dap` executable is not found

[vogelsgesang]

Currently, if the lldb-dap executable is not found / not configured, there is no feedback displayed to the user. It would be more user-friendly if there would be an error message, also pointing towards instructions on how to configure the lldb-dap executable path

[llvmbot]

@llvm/issue-subscribers-lldb

Author: Adrian Vogelsgesang (vogelsgesang)

Currently, if the `lldb-dap` executable is not found / not configured, there is no feedback displayed to the user. It would be more user-friendly if there would be an error message, also pointing towards instructions on how to configure the `lldb-dap` executable path