Added a rudimentary error checking to RunProcess.

[igorbakshee]

Set up a rudimentary error handling. E.g., for a pip command not on $PATH:

$ pip
-bash: pip: command not found

the following used to give multiple errors and a sub-optimal result:

In[12]:= PythonPackageVersion[]
During evaluation of In[12]:= RunProcess::pnfd: Program pip not found. Check Environment["PATH"].
During evaluation of In[12]:= StringTrim::strse: String or list of strings expected at position 1 in StringTrim[$Failed].
Out[12]= StringTrim[$Failed]

and this gave a wrong result:

In[13]:= PythonPackageInstalledQ["xx"]
During evaluation of In[13]:= RunProcess::pnfd: Program pip not found. Check Environment["PATH"].
Out[13]= True

Now the non-string result from RunProcess is no longer fed to string-processing functions:

In[16]:= PythonPackageVersion[]
During evaluation of In[16]:= RunProcess::pnfd: Program pip not found. Check Environment["PATH"].
Out[16]= $Failed

In[17]:= PythonPackageInstalledQ["xx"]
During evaluation of In[17]:= RunProcess::pnfd: Program pip not found. Check Environment["PATH"].
Out[17]= False