get rid of some rare error classes, use RTError instead

docs/internals/frontends.rst

@@ -575,10 +575,6 @@ Errors
     ErrorWithTraceback rc: 2 traceback: yes
         Error: {}
 
-    ExtensionModuleError rc: 2 traceback: no
-        The Borg binary extension modules do not seem to be properly installed.
-    PythonLibcTooOld rc: 2 traceback: no
-        FATAL: this Python was compiled for a too old (g)libc and misses required functionality.
     Buffer.MemoryLimitExceeded rc: 2 traceback: no
         Requested buffer size {} is above the limit of {}.
     EfficientCollectionQueue.SizeUnderflow rc: 2 traceback: no

src/borg/helpers/checks.py

@@ -1,38 +1,31 @@
 import os
 
-from .errors import Error
+from .errors import RTError
 from ..platformflags import is_win32
 
 
-class PythonLibcTooOld(Error):
-    """FATAL: this Python was compiled for a too old (g)libc and misses required functionality."""
-
-
 def check_python():
     if is_win32:
         required_funcs = {os.stat}
     else:
         required_funcs = {os.stat, os.utime, os.chown}
     if not os.supports_follow_symlinks.issuperset(required_funcs):
-        raise PythonLibcTooOld
-
-
-class ExtensionModuleError(Error):
-    """The Borg binary extension modules do not seem to be properly installed."""
+        raise RTError("""FATAL: this Python was compiled for a too old (g)libc and misses required functionality.""")
 
 
 def check_extension_modules():
     from .. import platform, compress, crypto, item, chunker, hashindex
 
+    msg = """The Borg binary extension modules do not seem to be properly installed."""
     if hashindex.API_VERSION != "1.2_01":
-        raise ExtensionModuleError
+        raise RTError(msg)
     if chunker.API_VERSION != "1.2_01":
-        raise ExtensionModuleError
+        raise RTError(msg)
     if compress.API_VERSION != "1.2_02":
-        raise ExtensionModuleError
+        raise RTError(msg)
     if crypto.low_level.API_VERSION != "1.3_01":
-        raise ExtensionModuleError
+        raise RTError(msg)
     if item.API_VERSION != "1.2_01":
-        raise ExtensionModuleError
+        raise RTError(msg)
     if platform.API_VERSION != platform.OS_API_VERSION or platform.API_VERSION != "1.2_05":
-        raise ExtensionModuleError
+        raise RTError(msg)