make get-pip.py download failures more clear

curl dying ends up being a really unclear failure condition, and
hard to fingerprint in the gate. We should make this much more
explicit when we die.

Also, don't trust the upstream filename, because all the rest of
our logic would break if it changes anyway.

Change-Id: Ibc2a96b33471d24c597af0d7af896fb10523156f
diff --git a/tools/install_pip.sh b/tools/install_pip.sh
index d3f5e78..017e716 100755
--- a/tools/install_pip.sh
+++ b/tools/install_pip.sh
@@ -25,6 +25,7 @@
 FILES=$TOP_DIR/files
 
 PIP_GET_PIP_URL=https://bootstrap.pypa.io/get-pip.py
+LOCAL_PIP="$FILES/$(basename $PIP_GET_PIP_URL)"
 
 GetDistro
 echo "Distro: $DISTRO"
@@ -41,12 +42,11 @@
 
 
 function install_get_pip {
-    if [[ ! -r $FILES/get-pip.py ]]; then
-        (cd $FILES; \
-            curl -O $PIP_GET_PIP_URL; \
-        )
+    if [[ ! -r $LOCAL_PIP ]]; then
+        curl -o $LOCAL_PIP $PIP_GET_PIP_URL || \
+            die $LINENO "Download of get-pip.py failed"
     fi
-    sudo -E python $FILES/get-pip.py
+    sudo -E python $LOCAL_PIP
 }