Improve error message when opening invalid image data from clipboard

[fire-eggs]

System information:

• Windows OS version: Win7
• ImageGlass version: 7.6.4.30

To Reproduce

Steps to reproduce the behavior:

1. Copy some text into the clipboard [see additional context below].
2. Run imageglass
3. Open an image from the clipboard.

Actual behavior:

ImageGlass crashes.

Unhandled exception error details:

System.Exception: Base-64 file content is invalid.
   at ImageGlass.Heart.Photo.ConvertBase64ToBytes(String content)
   at ImageGlass.Heart.Photo.ConvertBase64ToBitmap(String content)
   at ImageGlass.frmMain.mnuMainOpenImageData_Click(Object sender, EventArgs e)

Expected behavior:

ImageGlass produces an error message to the effect of "clipboard does not contain image data".

Screenshots:

Additional context:

The text I happened to have in the clipboard at the time was:

System.ArgumentOutOfRangeException: startIndex cannot be larger than length of string.
Parameter name: startIndex
   at System.String.Substring(Int32 startIndex, Int32 length)
   at ImageGlass.frmMain.<mnuMainSaveAs_Click>d__133.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

[d2phap]

Done in the latest build of IG Moon https://imageglass.org/moon