Protect against launching the theorem prover multiple times

[pglvdm]

If one by mistake press the "start theorem prover" twice one gets an error like: "Only a single prover can be running at any time - stop the running prover before launching a new one". The IDE should catch this kind of problem. The error message is ok but it should not come out as an error in the Error Log

[ldcouto]

The launching is provided by Isabelle/Eclipse and this behavior comes from there as well.

Re-tagging as an enhancement.