debian: check for errors in preinst scripts.

Fixes maintainer-script-ignores-errors lintian warning.

debian/nginx-module-geoip.preinst

diff --git a/debian/nginx-module-geoip.preinst b/debian/nginx-module-geoip.preinst
index 33498d7..c7c8e97 100644
--- a/debian/nginx-module-geoip.preinst
+++ b/debian/nginx-module-geoip.preinst
@@ -1,5 +1,7 @@
 #!/bin/sh
 
+set -e
+
 case "$1" in
     install)
         cat <<BANNER

debian/nginx-module-image-filter.preinst

diff --git a/debian/nginx-module-image-filter.preinst b/debian/nginx-module-image-filter.preinst
index 6269778..13ca418 100644
--- a/debian/nginx-module-image-filter.preinst
+++ b/debian/nginx-module-image-filter.preinst
@@ -1,5 +1,7 @@
 #!/bin/sh
 
+set -e
+
 case "$1" in
     install)
         cat <<BANNER

debian/nginx-module-njs.preinst

diff --git a/debian/nginx-module-njs.preinst b/debian/nginx-module-njs.preinst
index 7bf8358..363b8d8 100644
--- a/debian/nginx-module-njs.preinst
+++ b/debian/nginx-module-njs.preinst
@@ -1,5 +1,7 @@
 #!/bin/sh
 
+set -e
+
 case "$1" in
     install)
         cat <<BANNER

debian/nginx-module-perl.preinst

diff --git a/debian/nginx-module-perl.preinst b/debian/nginx-module-perl.preinst
index 9db420f..798a7f4 100644
--- a/debian/nginx-module-perl.preinst
+++ b/debian/nginx-module-perl.preinst
@@ -1,5 +1,7 @@
 #!/bin/sh
 
+set -e
+
 case "$1" in
     install)
         cat <<BANNER

debian/nginx-module-xslt.preinst

diff --git a/debian/nginx-module-xslt.preinst b/debian/nginx-module-xslt.preinst
index 6bf94d3..99c6b86 100644
--- a/debian/nginx-module-xslt.preinst
+++ b/debian/nginx-module-xslt.preinst
@@ -1,5 +1,7 @@
 #!/bin/sh
 
+set -e
+
 case "$1" in
     install)
         cat <<BANNER