Though, thinking about it, I suppose I could do something like the
following (thanks *Andrew*):

<xsl:template match="/subscriptions/item[renewaldate]">
<p>Your renewal date for <xsl:value-of select="subscriptionname" /> is:
<xsl:value-of select="renewaldate" />.</p>
</xsl:template>

<xsl:template match="/subscriptions/item[not(renewaldate)]">
<p>Sorry, there has been an error...</p>
</xsl:template>

Feels weird though, because actions for the same element aren't being
kept "together" (within the same template), but it does look more useful
than a return.

<xsl:template match="/subscriptions/item">
  <xsl:choose>
     <xsl:when test="renewaldate">
        <p>Your renewal date for <xsl:value-of select="subscriptionname" /> is:
        <xsl:value-of select="renewaldate" />.</p>
    </xsl:when>
    <xsl:otherwise>
       <p>Sorry, there has been an error...</p>
    </xsl:otherwise>
</xsl:template>

Regards,
Mukul