Add handling of error when poetry.lock does not exist

Fixes #81

tox_poetry_installer/exceptions.py

@@ -11,6 +11,7 @@ All exceptions should inherit from the common base exception :exc:`ToxPoetryInst
    +-- LockedDepNotFoundError
    +-- ExtraNotFoundError
    +-- LockedDepsRequiredError
+   +-- LockfileParsingError
 
 """
 
@@ -41,3 +42,7 @@ class ExtraNotFoundError(ToxPoetryInstallerException):
 
 class LockedDepsRequiredError(ToxPoetryInstallerException):
     """Environment cannot specify unlocked dependencies when locked dependencies are required"""
+
+
+class LockfileParsingError(ToxPoetryInstallerException):
+    """Failed to load or parse the Poetry lockfile"""

tox_poetry_installer/hooks/tox_on_install.py

@@ -41,10 +41,16 @@ def tox_on_install(
 
     virtualenv = convert_virtualenv(tox_env)
 
+    try:
         if not poetry.locker.is_fresh():
             logger.warning(
                 f"The Poetry lock file is not up to date with the latest changes in {poetry.file}"
             )
+    except FileNotFoundError as err:
+        logger.error(f"Could not parse lockfile: {err}")
+        raise exceptions.LockfileParsingError(
+            f"Could not parse lockfile: {err}"
+        ) from err
 
     try:
         if tox_env.conf["require_locked_deps"] and tox_env.conf["deps"].lines():