Clarify the Readme a bit

README.md

@@ -86,6 +86,13 @@ Add the following to your `.bashrc` (or `.profile` on Mac):
 ```bash
 function _update_ps1() {
     PS1="$($GOPATH/bin/powerline-go -error $? -jobs $(jobs -p | wc -l))"
+
+    # Uncomment the following line to automatically clear errors after showing
+    # them once. This not only clears the error for powerline-go, but also for
+    # everything else you run in that shell. Don't enable this if you're not
+    # sure this is what you want.
+
+    #set "?"
 }
 
 if [ "$TERM" != "linux" ] && [ -f "$GOPATH/bin/powerline-go" ]; then
@@ -102,6 +109,13 @@ Add the following to your `.zshrc`:
 ```bash
 function powerline_precmd() {
     PS1="$($GOPATH/bin/powerline-go -error $? -jobs ${${(%):%j}:-0})"
+
+    # Uncomment the following line to automatically clear errors after showing
+    # them once. This not only clears the error for powerline-go, but also for
+    # everything else you run in that shell. Don't enable this if you're not
+    # sure this is what you want.
+
+    #set "?"
 }
 
 function install_powerline_precmd() {
@@ -113,7 +127,7 @@ function install_powerline_precmd() {
   precmd_functions+=(powerline_precmd)
 }
 
-if [ "$TERM" != "linux" ]; then
+if [ "$TERM" != "linux" ] && [ -f "$GOPATH/bin/powerline-go" ]; then
     install_powerline_precmd
 fi
 ```