Tweak _ConverterError reporting.

Chaining an empty _ConverterError doesn't add useful info; instead,
print out the output stream.

e.g. modify _GSConverter to use the invalid `-sDEVICE=1234`; previously
`_GSConverter()("foo", "bar")` would give:
```
Traceback (most recent call last):
  File ".../matplotlib/testing/compare.py", line 110, in __call__
    self._read_until(b"\nGS")
  File ".../matplotlib/testing/compare.py", line 95, in _read_until
    raise _ConverterError
matplotlib.testing.compare._ConverterError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/antony/src/extern/matplotlib/lib/matplotlib/testing/compare.py", line 112, in __call__
    raise OSError("Failed to start Ghostscript") from err
OSError: Failed to start Ghostscript
```
or, if just passing buf as parameter when constructing the
_ConverterError:
```
Traceback (most recent call last):
  File ".../matplotlib/testing/compare.py", line 110, in __call__
    self._read_until(b"\nGS")
  File ".../matplotlib/testing/compare.py", line 95, in _read_until
    raise _ConverterError(buf)
matplotlib.testing.compare._ConverterError: bytearray(b'GPL Ghostscript 9.55.0 (2021-09-27)\nCopyright (C) 2021 Artifex Software, Inc.  All rights reserved.\nThis software is supplied under the GNU AGPLv3 and comes with NO WARRANTY:\nsee the file COPYING for details.\nUnknown device: 1234\n')

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File ".../matplotlib/testing/compare.py", line 112, in __call__
    raise OSError("Failed to start Ghostscript") from err
OSError: Failed to start Ghostscript
```
and now it gives:
```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File ".../matplotlib/testing/compare.py", line 112, in __call__
    raise OSError("Failed to start Ghostscript:\n\n" + err.args[0]) from None
OSError: Failed to start Ghostscript:

GPL Ghostscript 9.55.0 (2021-09-27)
Copyright (C) 2021 Artifex Software, Inc.  All rights reserved.
This software is supplied under the GNU AGPLv3 and comes with NO WARRANTY:
see the file COPYING for details.
Unknown device: 1234
```

Author: anntzer

lib/matplotlib/testing/compare.py

@@ -92,7 +92,7 @@ def _read_until(self, terminator):
         while True:
             c = self._proc.stdout.read(1)
             if not c:
-                raise _ConverterError
+                raise _ConverterError(os.fsdecode(bytes(buf)))
             buf.extend(c)
             if buf.endswith(terminator):
                 return bytes(buf)
@@ -109,7 +109,8 @@ def __call__(self, orig, dest):
             try:
                 self._read_until(b"\nGS")
             except _ConverterError as err:
-                raise OSError("Failed to start Ghostscript") from err
+                raise OSError(
+                    "Failed to start Ghostscript:\n\n" + err.args[0]) from None
 
         def encode_and_escape(name):
             return (os.fsencode(name)
@@ -172,8 +173,9 @@ def __call__(self, orig, dest):
             try:
                 self._read_until(terminator)
             except _ConverterError as err:
-                raise OSError("Failed to start Inkscape in interactive "
-                              "mode") from err
+                raise OSError(
+                    "Failed to start Inkscape in interactive mode:\n\n"
+                    + err.args[0]) from err
 
         # Inkscape's shell mode does not support escaping metacharacters in the
         # filename ("\n", and ":;" for inkscape>=1).  Avoid any problems by