Disk space checks should come earlier and use MB in the message

constructor/header.sh

@@ -21,6 +21,16 @@ if ! echo "$0" | grep '\.sh$' > /dev/null; then
     return 1
 fi
 
+total_installation_size_kb="__TOTAL_INSTALLATION_SIZE_KB__"
+free_disk_space_bytes="$(df -Pk "$PREFIX" | tail -n 1 | awk '{print $4}')"
+free_disk_space_kb="$((free_disk_space_bytes / 1024))"
+free_disk_space_kb_with_buffer="$((free_disk_space_kb - 50 * 1024))"  # add 50MB of buffer
+if [ "$free_disk_space_kb_with_buffer" -lt "$total_installation_size_kb" ]; then
+    printf "ERROR: Not enough free disk space. Only %s MB are available, but %s MB are required (leaving a 50 MB buffer).\\n" \
+        "$((free_disk_space_kb / 1024))" "$((total_installation_size_kb / 1024))" >&2
+    exit 1
+fi
+
 #if osx and min_osx_version
 min_osx_version="__MIN_OSX_VERSION__"
 system_osx_version=$(SYSTEM_VERSION_COMPAT=0 sw_vers -productVersion)
@@ -394,15 +404,6 @@ if ! mkdir -p "$PREFIX"; then
     exit 1
 fi
 
-total_installation_size_kb="__TOTAL_INSTALLATION_SIZE_KB__"
-free_disk_space_bytes="$(df -Pk "$PREFIX" | tail -n 1 | awk '{print $4}')"
-free_disk_space_kb="$((free_disk_space_bytes / 1024))"
-free_disk_space_kb_with_buffer="$((free_disk_space_bytes - 100 * 1024))"  # add 100MB of buffer
-if [ "$free_disk_space_kb_with_buffer" -lt "$total_installation_size_kb" ]; then
-    printf "ERROR: Not enough free disk space: %s < %s\\n" "$free_disk_space_kb_with_buffer" "$total_installation_size_kb" >&2
-    exit 1
-fi
-
 # pwd does not convert two leading slashes to one
 # https://github.com/conda/constructor/issues/284
 PREFIX=$(cd "$PREFIX"; pwd | sed 's@//@/@')