dependencies: Improve error message when variable is not found

The current error message does not tell which variable is not found and
prints internal representation of the dependency object:

  ERROR: Could not get pkg-config variable and no default provided for <PkgConfigDependency glib-2.0: True None>

mesonbuild/dependencies/base.py

@@ -347,7 +347,7 @@ class InternalDependency(Dependency):
         val = self.variables.get(internal, default_value)
         if val is not None:
             return val
-        raise DependencyException(f'Could not get an internal variable and no default provided for {self!r}')
+        raise DependencyException(f'Could not get an internal variable {internal} and no default provided')
 
     def generate_link_whole_dependency(self) -> Dependency:
         from ..build import SharedLibrary, CustomTarget, CustomTargetIndex

mesonbuild/dependencies/cmake.py

@@ -637,7 +637,7 @@ class CMakeDependency(ExternalDependency):
                 return ';'.join(v)
         if default_value is not None:
             return default_value
-        raise DependencyException(f'Could not get cmake variable and no default provided for {self!r}')
+        raise DependencyException(f'Could not get {self.name} cmake variable {cmake} and no default provided')
 
 
 class CMakeDependencyFactory:

mesonbuild/dependencies/configtool.py

@@ -170,4 +170,4 @@ class ConfigToolDependency(ExternalDependency):
                 return variable
         if default_value is not None:
             return default_value
-        raise DependencyException(f'Could not get config-tool variable and no default provided for {self!r}')
+        raise DependencyException(f'Could not get {self.name} variable {configtool} using {self.tool_name} and no default provided')

mesonbuild/dependencies/pkgconfig.py

@@ -586,4 +586,4 @@ class PkgConfigDependency(ExternalDependency):
                 pass
         if default_value is not None:
             return default_value
-        raise DependencyException(f'Could not get pkg-config variable and no default provided for {self!r}')
+        raise DependencyException(f'Could not get {self.name} pkg-config variable {pkgconfig} and no default provided')