Do not cancel shell init on mcfly init failure

The shell initialiation codes contain "return 1" at the top level.
However, this causes the cancellation of the entire shell
initialization because those codes are supposed to be evaluated as
e.g. `eval "$(mcfly init bash)"`.  The invocation of `return` in the
eval'ed string takes an effect in the caller context of `eval`.

mcfly.bash

@@ -1,8 +1,13 @@
 #!/bin/bash
 
-# Ensure stdin is a tty
-# Avoid loading this file more than once
-if [[ -t 0 ]] && [[ "$__MCFLY_LOADED" != "loaded" ]]; then
+function mcfly_initialize {
+  unset -f "${FUNCNAME[0]}"
+
+  # Ensure stdin is a tty
+  [[ -t 0 ]] || return 0
+
+  # Avoid loading this file more than once
+  [[ "$__MCFLY_LOADED" != "loaded" ]] || return 0
   __MCFLY_LOADED="loaded"
 
   # Setup MCFLY_HISTFILE and make sure it exists.
@@ -120,5 +125,5 @@ if [[ -t 0 ]] && [[ "$__MCFLY_LOADED" != "loaded" ]]; then
       fi
     fi
   fi
-
-fi
+}
+mcfly_initialize

mcfly.zsh

@@ -1,7 +1,8 @@
 #!/bin/zsh
 
-# Ensure stdin is a tty
-if [[ -o interactive ]]; then
+() {
+  # Ensure stdin is a tty
+  [[ -o interactive ]] || return 0
 
   # Setup MCFLY_HISTFILE and make sure it exists.
   export MCFLY_HISTFILE="${HISTFILE:-$HOME/.zsh_history}"
@@ -100,5 +101,4 @@ if [[ -o interactive ]]; then
     zle -N mcfly-history-widget
     bindkey '^R' mcfly-history-widget
   fi
-
-fi
+}