Don't add file content to the exception when requesting XML documents, if the content is not XML

[josegar74]

When an http request is made that expects an XML document, if this is not the case, the content of the file is added to the exception. If the file is very large or is a binary file, the log is overloaded.

Adding the file content to the exception / log seems useless. The pull request adds a generic message, reporting the problematic URL.

For remote ATOM Feeds, the exception is added to the result report of the collector, producing an invalid response.

Checklist

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