Since you are using XSLT 2.0, then you should also add an as
attribute to your xsl:param in the template. For example (you may have to use a different as value depending on what you need the resulting number to be, such as if you'll have values that have decimals; you would also need to correct the tunnel value, per Michael Kay's point):
<xsl:param name="errorCount" tunnel="yes" as="xs:integer" />
The transform would fail if it couldn't be converted to the as type (in this case, an integer). Eero's solution may look cleaner because you'll still have to check to see if the value is greater than zero. But, because you are using XSLT 2.0, the best practice is to type your parameters/variables.