portal_transform api return error message instead of raising error when coverting pdf to plain text

[jaroel]

From @nngu6036 on August 1, 2017 6:6

try:
    transforms = getToolByName(context, 'portal_transforms')
    stream = transforms.convertTo(
            'text/plain',
            output_data,
            mimetype='application/pdf')
    if stream:
        text = stream.getData()
except:
    return ''

For some PDF, the return values is "Syntax Error: Expected the optional content group list, but wasn’t able to find it, or it isn’t an Array Permission Error: Copying of text from this document is not allowed.“,

Copied from original issue: plone/plone.api#378