Fix "unused result" warnings for several system calls.

src/file.cc

@@ -100,7 +100,9 @@ String compact_path(StringView filename)
     String real_filename = real_path(filename);
 
     char cwd[1024];
-    getcwd(cwd, 1024);
+    if (!::getcwd(cwd, 1024))
+        throw runtime_error(format("unable to get the current working directory (errno: {})", ::strerror(errno)));
+
     String real_cwd = real_path(cwd) + "/";
     if (prefix_match(real_filename, real_cwd))
         return real_filename.substr(real_cwd.length()).str();
@@ -223,7 +225,8 @@ void write_buffer_to_fd(Buffer& buffer, int fd)
         eoldata = "\n";
 
     if (buffer.options()["BOM"].get<ByteOrderMark>() == ByteOrderMark::Utf8)
-        ::write(fd, "\xEF\xBB\xBF", 3);
+        if (::write(fd, "\xEF\xBB\xBF", 3) < 0)
+            throw runtime_error(format("unable to write data to the buffer (fd: {}; errno: {})", fd, ::strerror(errno)));
 
     for (LineCount i = 0; i < buffer.line_count(); ++i)
     {

src/remote.cc

@@ -532,7 +532,8 @@ void RemoteClient::write_next_key()
 void send_command(StringView session, StringView command)
 {
     int sock = connect_to(session);
-    ::write(sock, command.data(), (int)command.length());
+    if (::write(sock, command.data(), (int)command.length()) < 0)
+        throw runtime_error(format("unable to write data to socket (fd: {}; errno: {})", sock, ::strerror(errno)));
     close(sock);
 }
 

src/shell_manager.cc

@@ -29,7 +29,10 @@ namespace
 
 struct Pipe
 {
-    Pipe() { ::pipe(m_fd); }
+    Pipe() {
+        if (::pipe(m_fd) < 0)
+            throw runtime_error(format("unable to create pipe (fds: {}/{}; errno: {})", m_fd[0], m_fd[1], ::strerror(errno)));
+    }
     ~Pipe() { close_read_fd(); close_write_fd(); }
 
     int read_fd() const { return m_fd[0]; }