Refine test cases by checking message output

plug.vim

@@ -587,7 +587,7 @@ function! s:do(pull, force, todo)
     let installed = has_key(s:update.new, name)
     let updated = installed ? 0 :
       \ (a:pull && !empty(s:system_chomp('git log --pretty=format:"%h" "HEAD...HEAD@{1}"', spec.dir)))
-    if a:force || installed || updated 
+    if a:force || installed || updated
       execute 'cd' s:esc(spec.dir)
       call append(3, '- Post-update hook for '. name .' ... ')
       let type = type(spec.do)

test/workflow.vader

@@ -627,7 +627,8 @@ Execute (Single uninstalled executable):
   call plug#begin()
   Plug 'junegunn/vim-easy-align', { 'needs': 'does-not-exist' }
   call plug#end()
-  PlugInstall!
+  PlugInstall
+  AssertExpect 'Please install does-not-exist first.', 0
   q
   Assert !isdirectory(g:plugs['vim-easy-align'].dir),
     \ 'vim-easy-align should not exist'
@@ -636,7 +637,8 @@ Execute (Multiple uninstalled executables):
   call plug#begin()
   Plug 'junegunn/vim-easy-align', { 'needs': ['does-not-exist', 'also-non-existent'] }
   call plug#end()
-  PlugInstall!
+  PlugInstall
+  AssertExpect 'Please install does-not-exist, also-non-existent first.', 0
   q
   Assert !isdirectory(g:plugs['vim-easy-align'].dir),
     \ 'vim-easy-align should not exist'
@@ -645,7 +647,8 @@ Execute (Multiple uninstalled/installed executables):
   call plug#begin()
   Plug 'junegunn/vim-easy-align', { 'needs': ['does-not-exist', 'sh'] }
   call plug#end()
-  PlugInstall!
+  PlugInstall
+  AssertExpect 'Please install does-not-exist first.', 0
   q
   Assert !isdirectory(g:plugs['vim-easy-align'].dir),
     \ 'vim-easy-align should not exist'