Do not use SystemError

The docs for system error are clear:
```
>>> print(SystemError.__doc__)
Internal error in the Python interpreter.

Please report this to the Python maintainer, along with the traceback,
the Python version, and the hardware/OS platform and version.
```
None of these errors justify contacting a python maintainer.

`OSError` was probably intended, although a few of these make more sense as `ValueError`.

bin/test_executable.py

@@ -16,7 +16,7 @@ def test_executable(path):
             with open(path, 'r') as f:
                 if f.readline()[:2] != "#!":
                     exn_msg = "File at " + path + " either should not be executable or should have a shebang line"
-                    raise SystemError(exn_msg)
+                    raise OSError(exn_msg)
     else:
         for file in os.listdir(path):
             test_executable(os.path.join(path, file))

sympy/printing/preview.py

@@ -126,15 +126,15 @@ def preview(expr, output='png', viewer=None, euler=True, packages=(),
             try:
                 candidate_viewers = candidates[output]
             except KeyError:
-                raise SystemError("Invalid output format: %s" % output)
+                raise ValueError("Invalid output format: %s" % output) from None
 
             for candidate in candidate_viewers:
                 path = shutil.which(candidate)
                 if path is not None:
                     viewer = path
                     break
             else:
-                raise SystemError(
+                raise OSError(
                     "No viewers found for '%s' output format." % output)
     else:
         if viewer == "StringIO":
@@ -147,7 +147,7 @@ def preview(expr, output='png', viewer=None, euler=True, packages=(),
                 raise ValueError("outputbuffer has to be a BytesIO "
                                  "compatible object if viewer=\"BytesIO\"")
         elif viewer not in special and not shutil.which(viewer):
-            raise SystemError("Unrecognized viewer: %s" % viewer)
+            raise OSError("Unrecognized viewer: %s" % viewer)
 
 
     if preamble is None:
@@ -217,7 +217,7 @@ def preview(expr, output='png', viewer=None, euler=True, packages=(),
             try:
                 cmd_variants = commandnames[output]
             except KeyError:
-                raise SystemError("Invalid output format: %s" % output)
+                raise ValueError("Invalid output format: %s" % output) from None
 
             # find an appropriate command
             for cmd_variant in cmd_variants:
@@ -288,7 +288,7 @@ def preview(expr, output='png', viewer=None, euler=True, packages=(),
                 from pyglet.image.codecs.png import PNGImageDecoder
                 img = image.load(join(workdir, src), decoder=PNGImageDecoder())
             else:
-                raise SystemError("pyglet preview works only for 'png' files.")
+                raise ValueError("pyglet preview works only for 'png' files.")
 
             offset = 25