Better error messages if hook script is not found

Currently, edalize raises an Exception if a hook script cannot be
executed because it is not found:

```
Traceback (most recent call last):
  File "/home/philipp/.local/bin/fusesoc", line 11, in <module>
    load_entry_point('fusesoc', 'console_scripts', 'fusesoc')()
  File "/home/philipp/src/fusesoc/fusesoc/main.py", line 583, in main
    args.func(cm, args)
  File "/home/philipp/src/fusesoc/fusesoc/main.py", line 268, in run
    flags, args.system_name, args.system, args.backendargs, args.build_root)
  File "/home/philipp/src/fusesoc/fusesoc/main.py", line 357, in run_backend
    backend.build()
  File "/home/philipp/src/edalize/edalize/edatool.py", line 121, in build
    self.build_post()
  File "/home/philipp/src/edalize/edalize/edatool.py", line 133, in build_post
    self._run_scripts(self.hooks['post_build'])
  File "/home/philipp/src/edalize/edalize/edatool.py", line 283, in _run_scripts
    env = _env)
  File "/usr/lib64/python3.7/subprocess.py", line 342, in check_call
    retcode = call(*popenargs, **kwargs)
  File "/usr/lib64/python3.7/subprocess.py", line 323, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib64/python3.7/subprocess.py", line 775, in __init__
    restore_signals, start_new_session)
  File "/usr/lib64/python3.7/subprocess.py", line 1522, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: '../../../../../test/fpga_error_check.sh': '../../../../../test/fpga_error_check.sh'
```

After applying this patch, the user gets a more helpful error message:

```
INFO: Running post_build script fpga_error_check
DEBUG: Environment: {'SHELL': '/bin/bash', ... }
DEBUG: Working directory: /some/path/sim-verilator
ERROR: Failed to build xxx:xxx:xxx:xxx : Unable to run post_build script 'fpga_error_check': [Errno 2] No such file or directory: '../../../../../test/fpga_error_check.sh': '../../../../../test/fpga_error_check.sh'
```

Note that the INFO and DEBUG output relies on
olofk/fusesoc#307.
The Environment debug line can be rather long (depending on the shell
environment), but is very helpful for debugging script errors. Trading
off a "nice log" for a "useful log" here.

edalize/edatool.py

@@ -122,15 +122,15 @@ def build(self):
 
     def build_pre(self):
         if 'pre_build' in self.hooks:
-            self._run_scripts(self.hooks['pre_build'])
+            self._run_scripts(self.hooks['pre_build'], 'pre_build')
 
     def build_main(self):
         logger.info("Building");
         self._run_tool('make')
 
     def build_post(self):
         if 'post_build' in self.hooks:
-            self._run_scripts(self.hooks['post_build'])
+            self._run_scripts(self.hooks['post_build'], 'post_build')
 
     def run(self, args):
         logger.info("Running")
@@ -141,14 +141,14 @@ def run(self, args):
     def run_pre(self, args):
         self.parse_args(args, self.argtypes)
         if 'pre_run' in self.hooks:
-            self._run_scripts(self.hooks['pre_run'])
+            self._run_scripts(self.hooks['pre_run'], 'pre_run')
 
     def run_main(self):
         pass
 
     def run_post(self):
         if 'post_run' in self.hooks:
-            self._run_scripts(self.hooks['post_run'])
+            self._run_scripts(self.hooks['post_run'], 'post_run')
 
     def parse_args(self, args, paramtypes):
         if self.parsed_args:
@@ -271,19 +271,24 @@ def __init__(self, name, file_type, logical_name):
     def _param_value_str(self, param_value, str_quote_style="", bool_is_str=False):
         return jinja_filter_param_value_str(param_value, str_quote_style, bool_is_str)
 
-    def _run_scripts(self, scripts):
+    def _run_scripts(self, scripts, hook_name):
         for script in scripts:
             _env = self.env.copy()
             if 'env' in script:
                 _env.update(script['env'])
-            logger.info("Running " + script['name'])
+            logger.info("Running {} script {}".format(hook_name, script['name']))
+            logger.debug("Environment: " + str(_env))
+            logger.debug("Working directory: " + self.work_root)
             try:
                 subprocess.check_call(script['cmd'],
                                       cwd = self.work_root,
                                       env = _env)
+            except FileNotFoundError as e:
+                msg = "Unable to run {} script '{}': {}"
+                raise RuntimeError(msg.format(hook_name, script['name'], str(e)))
             except subprocess.CalledProcessError as e:
-                msg = "'{}' exited with error code {}"
-                raise RuntimeError(msg.format(script['name'], e.returncode))
+                msg = "{} script '{}' exited with error code {}"
+                raise RuntimeError(msg.format(hook_name, script['name'], e.returncode))
 
     def _run_tool(self, cmd, args=[]):
         logger.debug("Running " + cmd)