Correctly throw an exception if we fail to read in file.

`FileReader` excepts that `readAt` throws an exception is something goes
wrong. So `readAt` must throw an exception instead of returning `-1`.

src/fs_unix.cpp

@@ -47,8 +47,11 @@ zsize_t FD::readAt(char* dest, zsize_t size, offset_t offset) const
  errno = 0;
  while (size_to_read > 0) {
  auto size_read = PREAD(m_fd, dest, size_to_read, current_offset);
+ if (size_read == 0) {
+ throw std::runtime_error("Cannot read past the end of the file");
+ }
  if (size_read == -1) {
- return zsize_t(-1);
+ throw std::runtime_error("Cannot read file");
  }
  size_to_read -= size_read;
  current_offset += size_read;

src/fs_windows.cpp

@@ -69,7 +69,7 @@ FD::~FD()
 zsize_t FD::readAt(char* dest, zsize_t size, offset_t offset) const
 {
  if (!mp_impl)
- return zsize_t(-1);
+ throw std::runtime_error("FD is not open");
  EnterCriticalSection(&mp_impl->m_criticalSection);
  LARGE_INTEGER off;
  off.QuadPart = offset.v;
@@ -88,7 +88,7 @@ zsize_t FD::readAt(char* dest, zsize_t size, offset_t offset) const
  return size;
 err:
  LeaveCriticalSection(&mp_impl->m_criticalSection);
- return zsize_t(-1);
+ throw std::runtime_error("Cannot read");
 }
 
 bool FD::seek(offset_t offset)