More informative errors when providing wrong chromosome name or invalid coordinates

[dgruano]

In the following snippet

with pyBigWig.open(file) as bw:
  data = bw.values(chromosome, start, end, numpy=True)

If the chromosome value is not present in the bigWig chromosomes, the following error is raised:

RuntimeError: Invalid interval bounds!

However, this error suggests there is a problem with the coordinates, even if it is not the case.

It may be a useful change to separate the handling of both errors, by adding something like
RuntimeError: Invalid chromosome name!: {chromosome}.

Other improvement I can think of is printing the problematic coordinates, such as
RuntimeError: Invalid interval bounds! start: {start}, end: {end}, chromosome: {chromosome}, length: {length}

If useful, I could submit a PR.

This issue is related to #49 , #124 and #152