Add input validation for out of bounds column numbers

[pkerpedjiev]

When I run cooler cload -c1 1 -p1 2 -c2 4 - p2 5 .. on a file with four columns:

galactic        1601634 galactic        1125339
galactic        1601743 galactic        1135340
galactic        1597121 galactic        1126359
galactic        1598993 galactic        1135185
galactic        1597588 galactic        1125826

The error I get is:

Traceback (most recent call last):
  File "/Users/peter/miniconda3/envs/cenv3/bin/cooler", line 11, in <module>
    load_entry_point('cooler', 'console_scripts', 'cooler')()
  File "/Users/peter/miniconda3/envs/cenv3/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/Users/peter/miniconda3/envs/cenv3/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/Users/peter/miniconda3/envs/cenv3/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/peter/miniconda3/envs/cenv3/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/peter/miniconda3/envs/cenv3/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/peter/miniconda3/envs/cenv3/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/Users/peter/projects/cooler.mirnylab/cooler/cli/cload.py", line 409, in pairs
    ensure_sorted=False
  File "/Users/peter/projects/cooler.mirnylab/cooler/io.py", line 510, in create_from_unordered
    create(cool_uri, bins, chunks, columns=columns, dtype=dtype, **kwargs)
  File "/Users/peter/projects/cooler.mirnylab/cooler/io.py", line 286, in create
    file_path, target, meta.columns, iterable, h5opts, lock)
  File "/Users/peter/projects/cooler.mirnylab/cooler/_writer.py", line 203, in write_pixels
    for i, chunk in enumerate(iterable):
  File "/Users/peter/projects/cooler.mirnylab/cooler/_binning.py", line 701, in __iter__
    ignore_index=True)
  File "/Users/peter/miniconda3/envs/cenv3/lib/python3.6/site-packages/pandas/tools/merge.py", line 1334, in concat
    copy=copy)
  File "/Users/peter/miniconda3/envs/cenv3/lib/python3.6/site-packages/pandas/tools/merge.py", line 1367, in __init__
    raise ValueError('No objects to concatenate')
ValueError: No objects to concatenate

A much more informative error would be that the column I specified for p2 is invalid since the file only has 4 columns.

[nvictus]

Migrating to #209