Parquet: Add error message for no engine found (#18717)

* patquet.py: Add error message for no engine found

Give a better error message for engine="auto" case when none of the
engines were found installed.

* clean-up to prevent merge conflicts with other PR

Author: jorisvandenbossche

pandas/io/parquet.py

@@ -25,6 +25,11 @@ def get_engine(engine):
         except ImportError:
             pass
 
+        raise ImportError("Unable to find a usable engine; "
+                          "tried using: 'pyarrow', 'fastparquet'.\n"
+                          "pyarrow or fastparquet is required for parquet "
+                          "support")
+
     if engine not in ['pyarrow', 'fastparquet']:
         raise ValueError("engine must be one of 'pyarrow', 'fastparquet'")