Only show NSIS errors not warnings

Change-Id: I4b7a4cf0040fc0efc6859cc6d903eda2c47ce5a4
Reviewed-on: http://gerrit.dmdirc.com/1718
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>
Reviewed-by: Chris Smith <chris@dmdirc.com>

build-installer.xml

         <copy file="dist/DMDirc.jar"
             tofile="modules\installer\windows\files\DMDirc.jar"
             overwrite="true" />
-        <nsis script="modules\installer\windows\updater.nsi" verbosity="2">
+        <nsis script="modules\installer\windows\updater.nsi" verbosity="1">
             <define name="VERSION" value="${version.git.nsis}"/>
         </nsis>
-        <nsis script="modules\installer\windows\launcher.nsi" verbosity="2">
+        <nsis script="modules\installer\windows\launcher.nsi" verbosity="1">
             <define name="VERSION" value="${version.git.nsis}"/>
         </nsis>
-        <nsis script="modules\installer\windows\installer.nsi" verbosity="2">
+        <nsis script="modules\installer\windows\installer.nsi" verbosity="1">
             <define name="VERSION" value="${version.git.nsis}"/>
         </nsis>
         <delete dir="modules\installer\windows\files" />