fix set-azurermroledefinition bug

[darshanhs90]

Description

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[cormacpayne]

@darshanhs90 would you mind updating the Resources change log with a snippet of the changes being made in this PR?

[cormacpayne]

@darshanhs90 why is the try-catch being added?

[darshanhs90]

@cormacpayne Customers can add new scopes as part of the assignable scopes[irrespective of the position,either at the start or the end].
Assuming they add a new scope at the start,the getroledefinition call tries to get the roledefinition at the new scope and results in a Roledefinition doesnt exist. Hence when we add the try catch,we can check through all the assigned scopes if the Role defintiion exists or not.

[cormacpayne]

@darshanhs90 thanks for the explanation. This PR looks good once the change log is updated.

[cormacpayne]

@darshanhs90 looks like there are some merge conflicts after your other PR was merged. Would you mind taking a look?

[maddieclayton]

On demand run here: https://azuresdkci.westus2.cloudapp.azure.com/view/PowerShell/job/powershell-demand/213/