Handle multiple errors in same ErrorDialog

[HebaruSan]

Problem

If you start downloading multiple modules in GUI and more than one of them fail, you'll get an unhandled exception like this:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidOperationException: The form is already displayed as a modal dialog.
  at System.Windows.Forms.Form.ShowDialog (System.Windows.Forms.IWin32Window owner) [0x00081] in <f73fa304d089422f9e23b25394111843>:0 
  at System.Windows.Forms.Form.ShowDialog () [0x00000] in <f73fa304d089422f9e23b25394111843>:0 
  at CKAN.ErrorDialog.ShowErrorDialog (System.String text, System.Object[] args) [0x00071] in <ea98fd2118b84fc7ad1a2141a259ca95>:0 

Cause

In #2756 we started handling individual module download success or failure immediately, regardless of whether other downloads were still in progress. This opened the possibility of multiple calls to ErrorDialog.ShowErrorDialog without the error dialog being closed in between, and Form.ShowDialog throws the above exception when the form is already visible.

Changes

• Now ErrorDialog.ShowErrorDialog only calls ShowDialog if Visible is false, to avoid throwing the exception
• Now if ErrorDialog is already visible, additional messages will be appended to the main one
  • The text is cleared when the form is closed
  • The default text is now empty, so the first message won't be appended to "Error!" or "Fehler!"
• Now ErrorDialog is resizable and has anchored controls to fit the space
• Now ErrorDialog is wider by default
• Now ErrorDialog.ShowErrorDialog resizes the form to fit its content, up to a maximum height of 600 pixels, to make it easier to read the messages, just like Auto size yes no dialog #2729
• Now ErrorDialog will open centered on the main window instead of centered on the screen
• ErrorDialog no longer inherits from FormCompatibility because it doesn't help and was messing with the size

Fixes #2890.

[DasSkelett]

Works very well, now you can see the full beauty of multiple stack traces!