Include server command in failure to start message (#315)

Makes it easier to diagnose issues like #309

autoload/lsc/server.vim

@@ -284,7 +284,11 @@ function! lsc#server#register(filetype, config) abort
     let l:old_status = self.status
     if l:old_status ==# 'starting'
       let self.status= 'failed'
-      call lsc#message#error('Failed to initialize server: '.self.config.name)
+      let l:message = 'Failed to initialize server "'.self.config.name.'".'
+      if self.config.name != string(self.config.command)
+        let l:message .= ' Failing command is: '.string(self.config.command)
+      endif
+      call lsc#message#error(l:message)
     elseif l:old_status ==# 'exiting'
       let self.status= 'exited'
     elseif l:old_status ==# 'running'

test/integration/test/early_exit_test.dart

@@ -22,6 +22,6 @@ void main() {
   test('reports a failure to start', () async {
     await vim.edit('foo.txt');
     final messages = await vim.messages(1);
-    expect(messages, ['[lsc:Error] Failed to initialize server: \'false\'']);
+    expect(messages, ['[lsc:Error] Failed to initialize server "\'false\'".']);
   });
 }

test/integration/test/stderr_test.dart

@@ -28,7 +28,8 @@ void main() {
     final messages = await vim.messages(2);
     expect(messages, [
       '[lsc:Error] StdErr from some server: messagestderr',
-      '[lsc:Error] Failed to initialize server: some server'
+      '[lsc:Error] Failed to initialize server "some server". '
+          'Failing command is: [\'sh\', \'-c\', \'echo messagestderr >&2\']'
     ]);
   });
 
@@ -45,7 +46,8 @@ void main() {
     final messages = await vim.messages(2);
     expect(messages, [
       '"foo.txt" [New] --No lines in buffer--',
-      '[lsc:Error] Failed to initialize server: some server'
+      '[lsc:Error] Failed to initialize server "some server". '
+          'Failing command is: [\'sh\', \'-c\', \'echo messagestderr >&2\']'
     ]);
   });
 }