HyMacroExpansionError shouldn't truncate message

For example:

```
$ hy
hy 0.10.1 using CPython(default) 2.7.8 on Darwin
=> (defmacro hi [] (raise (TypeError "This message will be truncated")))
=> (hi)
  File "<input>", line 1, column 1

  (hi)
  ^--^
HyMacroExpansionError: `hi' message will be truncated
````
Merge pull request #694 from michaelmaltese/patch-1

HyMacroExpansionError shouldn't truncate message