Derive all exceptions correctly from base exceptions

Don't need to define __init__ and manually call the parent init. Doing
so messes up the error message you get by doing str(exception) because
it includes the current class name in it repeatedly.

mesonbuild/dependencies.py

@@ -30,8 +30,7 @@ from . import mesonlib
 from .environment import detect_cpu_family, for_windows
 
 class DependencyException(MesonException):
-    def __init__(self, *args, **kwargs):
-        MesonException.__init__(self, *args, **kwargs)
+    '''Exceptions raised while trying to find dependencies'''
 
 class Dependency():
     def __init__(self, type_name='unknown'):
@@ -170,9 +169,8 @@ class PkgConfigDependency(Dependency):
                 if not self.silent:
                     mlog.log(*found_msg)
                 if self.required:
-                    raise DependencyException(
-                        'Invalid version of a dependency, needed %s %s found %s.' %
-                        (name, not_found, self.modversion))
+                    m = 'Invalid version of dependency, need {!r} {!r} found {!r}.'
+                    raise DependencyException(m.format(name, not_found, self.modversion))
                 return
         found_msg += [mlog.green('YES'), self.modversion]
         if not self.silent:

mesonbuild/mesonlib.py

@@ -19,12 +19,10 @@ import platform, subprocess, operator, os, shutil, re
 from glob import glob
 
 class MesonException(Exception):
-    def __init__(self, *args, **kwargs):
-        Exception.__init__(self, *args, **kwargs)
+    '''Exceptions thrown by Meson'''
 
 class EnvironmentException(MesonException):
-    def __init__(self, *args, **kwargs):
-        super().__init__(*args, **kwargs)
+    '''Exceptions thrown while processing and creating the build environment'''
 
 class File:
     def __init__(self, is_built, subdir, fname):