Add USMAllocationError exception, use it for USM allocation failures

This PR addresses concern about use of non-descriptive RuntimeError
exception in earlier versions raised in issue gh-1158

```
In [1]: import dpctl

In [2]: import dpctl.memory as dpm

In [3]: dpm.MemoryUSMDevice( 1024 * 1024 * 1024 * 8)
---------------------------------------------------------------------------
USMAllocationError                        Traceback (most recent call last)
Input In [3], in <cell line: 1>()
----> 1 dpm.MemoryUSMDevice( 1024 * 1024 * 1024 * 8)

File ~/repos/dpctl/dpctl/memory/_memory.pyx:783, in dpctl.memory._memory.MemoryUSMDevice.__cinit__()
    781           SyclQueue queue=None, int copy=False):
        782 if (isinstance(other, numbers.Integral)):
	--> 783     self._cinit_alloc(alignment, <Py_ssize_t>other, b"device", queue)
	    784 else:
	        785     self._cinit_other(other)

File ~/repos/dpctl/dpctl/memory/_memory.pyx:201, in dpctl.memory._memory._Memory._cinit_alloc()
    199     self.queue = queue
        200 else:
	--> 201     raise USMAllocationError(
	    202         "USM allocation failed"
	        203     )

USMAllocationError: USM allocation failed

In [4]: quit
```

Author: oleksandr-pavlyk

dpctl/memory/__init__.py

@@ -32,12 +32,14 @@
     MemoryUSMDevice,
     MemoryUSMHost,
     MemoryUSMShared,
+    USMAllocationError,
     as_usm_memory,
 )
 
 __all__ = [
     "MemoryUSMDevice",
     "MemoryUSMHost",
     "MemoryUSMShared",
+    "USMAllocationError",
     "as_usm_memory",
 ]

dpctl/memory/_memory.pyx

@@ -71,11 +71,22 @@ import numpy as np
 __all__ = [
     "MemoryUSMShared",
     "MemoryUSMHost",
-    "MemoryUSMDevice"
+    "MemoryUSMDevice",
+    "USMAllocationError",
 ]
 
 include "_sycl_usm_array_interface_utils.pxi"
 
+class USMAllocationError(Exception):
+    """
+    An exception raised when Universal Shared Memory (USM) allocation
+    call returns a null pointer, signaling a failure to perform the allocation.
+    Some common reasons for allocation failure are:
+        * insufficient free memory to perform the allocation request
+        * allocation size exceeds the maximum supported by targeted backend
+    """
+    pass
+
 
 cdef void copy_via_host(void *dest_ptr, SyclQueue dest_queue,
                         void *src_ptr, SyclQueue src_queue, size_t nbytes):
@@ -177,7 +188,7 @@ cdef class _Memory:
                     with nogil: p = DPCTLmalloc_device(nbytes, QRef)
             else:
                 raise RuntimeError(
-                    "Pointer type is unknown: {}".format(
+                    "Pointer type '{}' is not recognized".format(
                         ptr_type.decode("UTF-8")
                     )
                 )
@@ -187,9 +198,13 @@ cdef class _Memory:
                 self.nbytes = nbytes
                 self.queue = queue
             else:
-                raise RuntimeError("Null memory pointer returned")
+                raise USMAllocationError(
+                    "USM allocation failed"
+                )
         else:
-            raise ValueError("Non-positive number of bytes found.")
+            raise ValueError(
+                "Number of bytes of request allocation must be positive."
+            )
 
     cdef _cinit_other(self, object other):
         cdef _Memory other_mem