XSEED Parser: better error message when trying to read a file that does not exist

[megies]

What does this PR do?

Shows a better error message when trying to read a local file with XSEED Parser but that path is not existing actually (e.g. mistyped filename etc).

The logic is quite convoluted and lots of different inputs are allowed of type str/bytes, so at the time the error gets raised it isn't immediately obvious if the input actually was a filename or not, so this is kind of a middle way to improve the message.

In [1]: from obspy.io.xseed import Parser
   ...: 
   ...: dlseedpath = '/home/code/my_dlseedfile.dlseed'
   ...: 
   ...: parser = Parser(dlseedpath)
---------------------------------------------------------------------------
OSError                                   Traceback (most recent call last)
Input In [1], in <cell line: 5>()
      1 from obspy.io.xseed import Parser
      3 dlseedpath = '/home/code/my_dlseedfile.dlseed'
----> 5 parser = Parser(dlseedpath)

File ~/git/obspy/obspy/io/xseed/parser.py:119, in Parser.__init__(self, data, debug, strict, compact)
    117 # if a file name is given, read it directly to the parser object
    118 if data:
--> 119     self.read(data)

File ~/miniconda3/envs/master/lib/python3.10/site-packages/decorator.py:232, in decorate.<locals>.fun(*args, **kw)
    230 if not kwsyntax:
    231     args, kw = fix(args, kw, sig)
--> 232 return caller(func, *(extras + args), **kw)

File ~/git/obspy/obspy/core/util/decorator.py:298, in map_example_filename.<locals>._map_example_filename(func, *args, **kwargs)
    296                 except IOError:
    297                     pass
--> 298 return func(*args, **kwargs)

File ~/git/obspy/obspy/io/xseed/parser.py:241, in Parser.read(self, data)
    238 if potential_filename_provided is not None \
    239         and not os.path.exists(potential_filename_provided):
    240     msg += '. If a filename was provided, the file does not exist.'
--> 241 raise IOError(msg)

OSError: First byte of data must be in [0-9<]. If a filename was provided, the file does not exist.

Why was it initiated? Any relevant Issues?

fixes #2686

PR Checklist

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