fix #303585 added return from restore session if failed read score

[igorkorsukov]

Resolves: https://musescore.org/en/node/303585

Problem: trying to get a view that is not exists and assert is triggered.
this happens when restoring a session if there is no score file

bool MuseScore::restoreSession(bool always)
{
...
    MasterScore* score = readScore(e.readElementText());
    if (score) {
        ...
        appendScore(score);  <= 1
   
    ...
   (tab3 == 0 ? tab1 : tab2)->initScoreView(idx1, vmag, magIdx, x, y);   <= 2
}

1 - Here should be added a view with score, but not added, because score is missing.
2 - Here get a view that does not exist

Fix: Added return from restoreSession if no score

Problem2: The procedure for opening the last file is done in 2 places - restoreSession and loadScores.

Therefore, if the session cannot be restored, then there will be another attempt to open the last file, and the user will be shown a second time a message stating that the file is missing (the first message was shown in restoreSession)

I don’t know why the opening of the last file was implemented in two places ... maybe for backward compatibility when the session functionality was introduced.

In order not to show the second message, if the file could not be read in restoreSession, then true is returned

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