[Et] Add file existence check just before running it

Manual scenario:

1. Install UGENE in "C:/Program Files/Unipro UGENE"
2. Launch UGENE. "UGENE.ini" is created
3. Uninstall UGENE
4. Install UGENE in "C:/Program Files/Unipro UGENE 1"
5. Launch UGENE
6. Open COI.aln
7. Align with MAFFT
   Expected: there are no errors in the log, MAFFT worked correctly
   Current: there are errors in the log

   [Tasks][TRACE][17:32] Promoting task {Run MAFFT alignment task} to 'Finished', error 'Can not remove folder for temporary files.'
   [Tasks][ERROR][17:32] Task {Run MAFFT alignment task} finished with error: Can not remove folder for temporary files.

This was due to the fact that the outdated .ini file contains paths to removed tools, for example

exToolId50\37.0=USUPP_MAFFT
exToolPath50\37.0=C:\\Program Files\\Unipro UGENE\\tools\\mafft\\mafft.bat
exToolIsValid50\37.0=true
exToolIsChecked50\37.0=true
exToolVersion50\37.0=7.475

The folder "C:/Program Files/Unipro UGENE" does not exist. I suggest adding a check for the existence of the tool file before running it. If there is no file, write about it in the log

Scenario

1. Copy mafft folder in sandbox_dir
2. Open UGENE, open Preferences...->External Tools
3. Set MAFFT path as "sandbox_dir/mafft/mafft.bat", wait for validating, click OK
4. Remove "sandbox_dir/mafft/"
5. Open COI.aln and Align with MAFFT
   Expected: the log contains errors like

   [Tasks][TRACE][17:32] Promoting task {Run MAFFT alignment task} to 'Finished', error 'file "sandbox_dir/mafft/mafft.bat" doesn't exist.'
   [Tasks][ERROR][17:32] Task {Run MAFFT alignment task} finished with error: file "sandbox_dir/mafft/mafft.bat" doesn't exist.

   Current: there are errors in the log

   [Tasks][TRACE][17:32] Promoting task {Run MAFFT alignment task} to 'Finished', error 'Can not remove folder for temporary files.' [Tasks][ERROR][17:32] Task {Run MAFFT alignment task} finished with error: Can not remove folder for temporary files.