Report an error from build_fixtures if can't find compiler
Report an error from build_fixtures if can't find compiler

Summary:
Running `build_fixtures` without building the Thrift compiler first gives a
not very helpful error message:
```
Traceback (most recent call last):
  File "thrift/compiler/test/build_fixtures", line 111, in <module>
    stderr=subprocess.PIPE,
  File "/usr/lib64/python2.7/subprocess.py", line 711, in __init__
    errread, errwrite)
  File "/usr/lib64/python2.7/subprocess.py", line 1327, in _execute_child
    raise child_exception
AttributeError: 'NoneType' object has no attribute 'rfind'
```

With this diff the following error message is produced:

```
error: cannot find the Thrift compiler (buck-out/gen/thrift/compiler/thrift)
```

Reviewed By: yfeldblum

Differential Revision: D9666918

fbshipit-source-id: d16fa0bd8762506b139560ea4bf59b631176e260

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift