Add set -u to frameworks and resources copy scripts

This adds `set -u` to both the frameworks and resources copy shell
scripts. This means that if the scripts attempt to reference any
variable that is undefined, they will fail.

lib/cocoapods/generator/copy_resources_script.rb

@@ -96,6 +96,7 @@ module Pod
       INSTALL_RESOURCES_FUNCTION = <<EOS
 #!/bin/sh
 set -e
+set -u
 
 mkdir -p "${TARGET_BUILD_DIR}/${UNLOCALIZED_RESOURCES_FOLDER_PATH}"
 

lib/cocoapods/generator/embed_frameworks_script.rb

@@ -37,6 +37,7 @@ module Pod
         script = <<-SH.strip_heredoc
           #!/bin/sh
           set -e
+          set -u
 
           echo "mkdir -p ${CONFIGURATION_BUILD_DIR}/${FRAMEWORKS_FOLDER_PATH}"
           mkdir -p "${CONFIGURATION_BUILD_DIR}/${FRAMEWORKS_FOLDER_PATH}"