read_bytes(): check if the request is satisfiable, change error msg

Resolve #61

The changed error message reflects more the meaning of the error and
less the specific implementation. All we can tell now is how many bytes
were requested and how many are actually _available_: we haven't
necessarily "gotten" any bytes yet, so it should not say "but got only
%d bytes".

kaitaistruct.py

@@ -292,11 +292,21 @@ def read_bytes(self, n):
                 "requested invalid %d amount of bytes" %
                 (n,)
             )
-        r = self._io.read(n)
-        if len(r) < n:
+
+        is_satisfiable = True
+        if self._io.seekable():
+            num_bytes_available = self.size() - self.pos()
+            is_satisfiable = (n <= num_bytes_available)
+
+        if is_satisfiable:
+            r = self._io.read(n)
+            num_bytes_available = len(r)
+            is_satisfiable = (n <= num_bytes_available)
+
+        if not is_satisfiable:
             raise EOFError(
-                "requested %d bytes, but got only %d bytes" %
-                (n, len(r))
+                "requested %d bytes, but only %d bytes available" %
+                (n, num_bytes_available)
             )
         return r