unhelpful error message when extract_dist fails

```
$ pyp2rpm ./dist/conu-0.0.1.dev0.tar.gz
Failed to extract data from setup.py script.
```

Ehm, that's not helpful.

So I tried `-s` switch and it produced more logs:
```
INFO Pyp2rpm initialized.
INFO Using /home/tt/rpmbuild/SOURCES as directory to save source.
INFO Local file: ./dist/conu-0.0.1.dev0.tar.gz copyed to /home/tt/rpmbuild/SOURCES/conu-0.0.1.dev0.tar.gz.
INFO Getting metadata from setup.py using SetupPyMetadataExtractor.
INFO Running extract_dist command with: /usr/bin/python3
ERROR Subprocess failed, stdout: , stderr: Traceback (most recent call last):
File "setup.py", line 68, in
install_requires=get_requirements()
File "setup.py", line 33, in get_requirements
with open("./requirements.txt") as fd:
FileNotFoundError: [Errno 2] No such file or directory: './requirements.txt'

ERROR Could not extract metadata with: /usr/bin/python3
INFO Running extract_dist command with: /usr/bin/python2
ERROR Subprocess failed, stdout: , stderr: Traceback (most recent call last):
File "setup.py", line 68, in
install_requires=get_requirements()
File "setup.py", line 33, in get_requirements
with open("./requirements.txt") as fd:
IOError: [Errno 2] No such file or directory: './requirements.txt'

ERROR Could not extract metadata with: /usr/bin/python2
Failed to extract data from setup.py script.
```

Would it be possible to either provide better error message or log more by default?